With Mitt Romney’s sweep six days ago of primaries in Maryland, Washington DC, and the battleground state of Wisconsin, the Republican frontrunner continues to build momentum in the Republican presidential nomination sweepstakes.  In the official RNC count of delegates, Romney just crossed the halfway mark to 1144 with 573 delegates, far ahead of Rick Santorum’s 202; counting all of the non-binding contests, Romney leads by a slightly larger amount, 656 to 272.  The next binding contests come two weeks from tomorrow, and Santorum will only be competitive in one — and Romney is loading up for a knockout blow there:

Republican Mitt Romney will air presidential campaign ads in most of Pennsylvania starting on Monday, when candidates return from their Easter break, a source close to the campaign said on Friday.

The $2.9 million advertising campaign will run in the Wilkes-Barre, Scranton, Erie, Altoona and Philadelphia media markets until the April 24 primary election.

Within a week, the ads will run in the Pittsburgh market. The Romney super PAC Restore Our Future is airing commercials on cable channels statewide.

The campaign’s ad buy reinforces the former Massachusetts governor’s determination to win the home state of ex-Sen. Rick Santorum, former House Speaker Newt Gingrich and U.S. Rep. Ron Paul of Texas. Santorum grew up in Butler County and owns a Penn Hills home, Gingrich spent childhood years in the Harrisburg area and Paul is a Green Tree native.

It’s not just advertising, either.  Since Romney should have little trouble winning in the other four states that go to the polls on the 24th — New York, Connecticut, Rhode Island, and Delaware, the latter of which is a winner-take-all primary — he can spend almost all of his time stumping in Pennsylvania.  That makes the fight uncomfortably similar to Michigan and Wisconsin, both of which Santorum was perceived as having a lead or an edge, and both of which fell to Romney in the end.

As the LA Times points out, Pennsylvania is Santorum’s firewall, at least on credibility:

The former senator from Pennsylvania has resurrected his career after a shattering 2006 reelection defeat. Dismissed as a hopeless long shot when his presidential run began, he’ll finish no worse than second for the Republican nomination. At 53, he’s one of the nation’s leading social conservatives, and his long-range future has never looked brighter.

But as he resumes a do-or-die Pennsylvania primary effort this week, he’ll need all his local connections and considerable campaign talents to survive what could be the final showdown of the 2012 GOP contest. Polls show him with a small lead over Mitt Romney, who’d like nothing more than to finish off his main rival in the April 24 election.

After a day spent traversing the state’s steeply eroded ridges, studded with redbud blossoms and trees just greening up, Santorum expressed satisfaction at returning to “familiar territory, where I can say, ‘No, no, there’s a shorter way to get there’ to the drivers.”

He’s all but said that a primary loss would end his candidacy. “We have to win here,” he told reporters during a stop at Bob’s Diner in Carnegie, a Pittsburgh suburb he represented as a young congressman in the early 1990s.

Unfortunately for Santorum, failure in Pennsylvania might spell the end of not just the current political campaign, but any future in electoral politics.  It took an extraordinary effort to bring Santorum back from that large 2006 defeat in his home state, and if it happens again in a Republican primary, it might take even longer to get past it.  That puts more pressure on Santorum to defend the state if he chooses to continue, and polls are showing mixed signals at how well he’s managing to do it at the moment.

The delegate math is becoming more and more untenable, too.  Santorum’s camp released an argument last week that the media (and the RNC, apparently) has the delegate allocations all wrong, and that he’s actually much closer to Romney.  In part, that argument was based on a claim made by Newt Gingrich in February that the RNC would force Florida and Arizona to proportionally allocate their delegates.  Even the RNC admits that they can’t dictate state allocations, and in any case they don’t appear inclined to try, as their own scorecard shows.

On Tuesday, 159 delegates will be allocated in the four other states, the vast majority of which will go to Romney, while Pennsylvania’s 72 delegates will probably be closely split between Romney and Santorum regardless of the order of the finish.  There is a good possibility that Romney can pad his delegate lead by 100 or so on the 24th in both counts.  May’s nine contests look more promising for Santorum in a vacuum, but with Romney having perhaps over 800 delegates overall or 700 in the bound-only count, this race will hit a tipping point soon regardless of whether Santorum can win a squeaker in Pennsylvania — and states like Indiana and Oregon will probably fall Romney’s way, while Texas’ big prize will be proportionally allocated anyway.

Plus, the Des Moines Register reports that the few superdelegates in the GOP have begun to move towards Romney:

The Associated Press has polled 114 of the 120 superdelegates, party members who can support any candidate for president they choose at the national convention in August, regardless of what happens in primaries or caucuses.

In the latest survey, conducted Tuesday to Friday, Romney has 35 endorsements, far more than anyone else but a modest figure for the apparent nominee. Gingrich has four endorsements, Santorum has two and Texas Rep. Ron Paul got one.

RNC members have been slowly embracing Romney. He picked up 11 new endorsements since the last AP survey a month ago, after the Super Tuesday contests. Over the course of the campaign, however, Romney methodically has added endorsements from every region of the country. In the U.S. territories, where voters help decide the nominee but can’t vote in the general election, Romney has dominated.

Santorum will have two weeks to decide whether he wants to roll the dice on his future in a Pennsylvania primary for a nomination in which the math looks almost impossible to overcome, or take his gains and play for the future while the opening for a gracious and party-building exit remains in play.  He’s overcome a lot of long odds to put himself in that position, and perhaps Santorum will feel that the risks are still worth taking.