Welcome to New Hampshire, where the fight for the establishment lane of the GOP presidential primary is turning into a circular firing squad.

As the year winds down, four Republicans have crisscrossed the state, pointing their attacks in all directions. And with less than 50 days until the first-in-the-nation primary, it’s only going to get worse.

Forget Iowa, which Cruz appears to be locking up. It’s New Hampshire that will cull this field. And with Christie, Bush and John Kasich making the Granite State the singular focus of their campaigns, and Rubio, should he lose Iowa, needing a top-tier finish, the fight to be the mainstream alternative to Cruz or Trump could end here…

If Trump wins the Feb. 9 primary a week after Cruz wins Iowa, only one or two candidates finishing behind him will likely have the momentum to carry on. If four or even five candidates split the vote of an establishment electorate that never coalesces behind one standard-bearer, there may be only hollow victories to declare on primary night because none will have the firepower to challenge Cruz or Trump in South Carolina.

Sen. Marco Rubio of Florida pushed back Wednesday against incoming GOP fire he’s getting on his U.S. Senate attendance record, saying the attacks are evidence that his opposition is getting increasingly desperate.

“Chris Christie is a funny guy, but he’s never in New Jersey – he’s gone half the time,” Mr. Rubio said on “Fox and Friends.”…

“He was totally opposed to it and didn’t go there to vote no,” Mr. Christie said. “Dude, show up to work and vote no, right? Just show up to work and vote no. And if you don’t want to, then quit.”…

“What’s happening, unfortunately, with some of these folks that you’ve mentioned – they’re growing increasingly desperate and increasingly nasty, and that’s okay,” Mr. Rubio said. “I’m not running against them – I’m running for president.”

@ChrisChristie has spent 219 days outside NJ yet runs on being governor. He may not even be a resident never mind Gov.

— Senator Rand Paul (@RandPaul) December 30, 2015

@ChrisChristie @marcorubio can lob accusations of who works less all they want. Fact is both are right and both should show up or resign

— Senator Rand Paul (@RandPaul) December 30, 2015

Ohio Gov. John Kasich compares former Florida Gov. Jeb Bush to obsolete technologies and the “Macarena” in a new presidential campaign ad…

To further hammer home the message, the video’s caption on YouTube reads: “He hasn’t been Gov for almost a decade, but Jeb Bush still loves the good ole days. Here are some of his other favs from back when…”

Both Kasich and Bush have hitched their presidential hopes to New Hampshire, the second-voting state in the primary system.

After repeatedly boasting that he has spent very little on his campaign so far, Donald Trump is set to begin spending $2 million a week on ads in early primary and caucus states.

Trump told reporters Tuesday that he will begin his first major paid media campaign, which he has refused to do despite large advertising buys from his Republican rivals and the super PACs supporting them, next month. Trump, who has no authorized outside group independently backing him, appears to be finally paying heed to polls that show the GOP race tightening in Iowa…

“Starting around January 4 we’re spending a lot of money,” he told reporters following a campaign event in Nashua, New Hampshire. “The press is hearing this for the first time, they’re probably gonna go crazy.”

Trump pledged on Tuesday to return fire with a flurry of negative ads if anyone aired spots attacking him — though he did not say whether any of his upcoming spots planned to be negative themselves.

Florida Sen. Marco Rubio’s supporters returned fire on a negative ad from Jeb Bush’s team, which released aggressive attacks against rival GOP presidential candidates on Tuesday.

Conservative Solutions PAC, which supports Rubio, is looking to capitalize off of Bush’s attacks as a fundraising opportunity. Warren Tompkins, the pro-Rubio PAC’s strategist, sent an email to supporters saying that Bush’s campaign has “tanked” and that the pro-Rubio super PAC needs donations to stop Bush’s team from ruining Rubio’s candidacy.

“[J]ust as we predicted, the attacks are coming fast and furious,” Tompkins wrote. “We might expect them from Hillary Clinton, as she sees Marco as the biggest threat to her winning the White House. We might expect them from Ted Cruz, as he sees Marco’s conservative record and strong foreign policy experience. But the attacks in Iowa are coming from Jeb Bush’s allies, from a candidate with no chance of winning in the state, or of winning the nomination, frankly.”

A top fundraiser for the super PAC supporting Bush, Right to Rise USA, laments that his group’s $30 million ad blitz hasn’t moved the needle.

“Our numbers don’t look great,” he concedes on the condition of anonymity. “I see our ads out there but it hasn’t moved the numbers. We’ve got to start doing a whole lot better than we have with polling.”

On Tuesday, Right to Rise unveiled its most direct television attack against Bush frenemy and GOP competitor Sen. Marco Rubio yet, slamming him for missed Senate hearings and votes. Another Right to Rise ad posted Tuesday compares Bush’s record to those of New Jersey Gov. Chris Christie and Ohio Gov. John Kasich. The moves signal that Bush’s allies believe Rubio primarily and Christie and Kasich secondarily stand squarely in their candidate’s path to becoming the establishment alternative to Donald Trump and Sen. Ted Cruz.

Christie’s RealClearPolitics average puts him in third place in New Hampshire. He’s second or within the margin of error of second place in the two polls conducted by local media (Boston Herald and WBUR). Christie has doubled his numbers from early November, and he’s climbing, while Rubio is basically flat over the past month.

Given Christie’s GOP establishment support in Iowa (today he added a former state party co-chair), it’s also not crazy to think he could finish third in the caucuses despite his currently dismal polling there.

Here’s a scenario that strikes me as not crazy:

Christie finishes third in Iowa, making him an attractive choice for anti-Cruz voters in New Hampshire, and for former Trump backers who now want an electable choice. This would put Christie in striking distance of a victory in New Hampshire.

Though polls show Bush is disliked even more than Trump, the former Florida governor made a point of placing his confidence in experienced Granite Staters — and reminded them they’d be partly to blame if Trump wins…

Rubio, who crossed paths with Bush twice on Tuesday, tried a different approach, poking good-natured fun at voters’ candidate lists.

“I hear people say, ‘Well, I’ve narrowed it down to eight!’” he said Wednesday in Franklin…

Christie and Kasich, both sitting governors (of New Jersey and Ohio, respectively) have adopted New Hampshire as their second home. Both are ahead of Bush, and in some polls Christie is essentially tied with Rubio behind Trump — though neither Kasich nor Christie have much of a campaign operation in other states…

“He’s the most personable presidential candidate I’ve ever met,” gushed 74-year-old Jim McConaha of Concord. “If Trump weren’t in the race, I think he’d be Number One, really.”

“You are the most important people in America right now,” Christie told a crowd at a campaign stop Saturday in Exeter, where numerous attendees cited national security as a top concern. “This country is in danger and we need a president who’ll protect the American people,” Christie said, channeling their concerns. He also needled Trump, though not by name, saying, “We’re not picking an entertainer-in-chief. We’re not casting a TV show. This is real.”…

“[Christie] could win New Hampshire, but then what? Compared with McCain 2008, he has a weaker long game and a stronger field of opponents,” said Jack Pitney, a political science professor at Claremont McKenna College.

The establishment’s nightmare scenario is that the center-right vote splits and gifts Trump the Granite State, leaving them with no traction and no alternative as the nomination contest moves to South Carolina, Nevada and a swath of southern states…

At a town hall Sunday, Kasich reflected on the unusually ugly primary contest, and had his own veiled jabs at Trump. “This is not that hard,” he told the crowd. “It’s the politics that’s hard… I’m not into loud statements. I’m not into banging my fists on a table. I’m not into dividing and name calling… I’m into leading. The yelling only goes so far.”

If Mitt Romney still hopes to stop Donald Trump and help nominate a more electable Republican, there’s an obvious place and time for him to act: the New Hampshire primary…

When Romney reluctantly took himself out of the running for 2016, he made clear he wasn’t happy with then-frontrunner Bush. He pointedly said: “I believe that one of our next generation of Republican leaders, one who may not be as well known as I am today, one who has not yet taken their message across the country, one who is just getting started, may well emerge as being better able to defeat the Democrat nominee. In fact, I expect and hope that to be the case.”

Christie must still be on Romney’s naughty list from his embrace of President Obama after Hurricane Sandy struck New Jersey. Some think that really helped Obama and hurt Romney in the final days of the 2012 campaign. Moreover, Romney decided against picking Christie as his running mate — favoring Paul Ryan instead — so it’d be difficult for him to argue that Christie was the best choice for president when he didn’t think he was the best choice for vice president…

That leaves Rubio…

Romney might be reluctant to endorse before Iowa, wanting to wait and see if Cruz crushes Trump there… and not wanting to be the target of insults from Trump and Cruz for over 30 days. But Romney knows that he has the potential to do what other GOP leaders would love to do, but don’t have the power to effect: Stop Trump, and help nominate someone with a better chance to win the general election and better suited to be president of the United States.

This is all to the good. With 34 days left to go until the first votes are tabulated in the Iowa caucuses, it’s time for the gloves to come off…

It’s possible they will tear each other down (with Cruz getting an assist on tearing down Rubio from Jeb Bush’s PAC), in which case Bush or Christie may be able to take advantage and secure support from those in the party who are eager to prevent a Trump nomination.

It’s also possible one of them will basically win the fight, and in winning the fight, will elevate himself (eventually) into a two-man race with Trump. In that case, given Trump’s extraordinarily high negatives even among Republicans, the winner of the Rubio-Cruz battle will probably take it all.

Even more important, having a real argument about issues will reveal the hollowness of Trump’s candidacy at exactly the right moment — when Republican voters cease seeing the GOP contest as a reality TV show and instead view their ballot as among the most important they will ever cast.