Not as important as Tim Scott’s support since Toomey’s not from an early state, but still significant big-picture. This isn’t just a case of Cruz losing out to Rubio on another endorsement among his Senate colleagues, it’s a litmus test of electability — or perceived electability. Why might a Republican from a purple state, whose seat is up this year by the way, prefer Rubio to Cruz or Trump? Hmmm:

The rollouts are designed to generate momentum for Rubio, who finished strongly in the Iowa caucuses and is hoping for a similar showing in New Hampshire. The Florida senator hopes to unify the Republican establishment around him and cut off avenues of support for his competitors in the mainstream lane of the GOP primary — Jeb Bush, Chris Christie, and John Kasich. Many in the party hierarchy would like to soon coalesce around one candidate and halt insurgents Donald Trump and Ted Cruz.

Toomey, a swing-state Republican and former president of the anti-tax Club for Growth, is facing a competitive reelection bid in 2016. Those close to the Pennsylvania senator say he’s deeply concerned about who represents the top of the ticket for the GOP.

It’s fine to argue that Rubio isn’t really more electable than Cruz, especially given how impressive Cruz’s Iowa organization was, but a guy whose political career depends on gauging who’ll win more votes in Pennsylvania this fall is betting that he is. Worth noting. The other significance of the Toomey announcement is that it comes just 24 hours after Scott’s announcement, suggesting that Team Marco has a string of big names lined up that it’ll roll out day by day before New Hampshire. Who’s next? Kelly Ayotte seems a sure bet to prefer Rubio — she’s hawkish, moderate, a Gang of Eight supporter, and, like Toomey, comes from a purple state — but she announced yesterday that she won’t endorse in the primary. It may be that her home state of New Hampshire is so influential in picking the nominee that she feared Cruz and Trump fans would take revenge on her in her own election this fall if she jumped in on Rubio’s side. If not Ayotte, though, then who? Rubio would love to complement Tim Scott’s tea-party cred with Mike Lee’s, but Lee is also a frequent Cruz ally and might not want to choose between them. Tom Cotton, another vocal hawk, seems like a good fit for Rubio and may feel free to endorse now that Huckabee’s out of the race and he doesn’t need to worry about offending Arkansas voters. Ben Sasse would be a nice get too, although Sasse was just in Iowa campaigning with Rubio and Cruz to stop Trump. I doubt he’d want to dilute his anti-Trump message by choosing between them so soon.

What about ol’ Mav? McCain said a few weeks ago that he wouldn’t endorse in the primary, although the opportunity to knife Cruz and Donald “He’s a hero because he got captured” Trump at this point might be irresistible. Then again, like Ayotte, McCain is up for reelection this year and doesn’t want to alienate Cruz fans in Arizona more than he already has. McCain’s pal Lindsey Graham is already committed to Jeb Bush, but here’s an interesting quote from today’s NYT story about Bush’s tribulations in New Hampshire:

Specifically, many Republicans — including some of his supporters and donors — said Tuesday that Mr. Bush must finish ahead of Mr. Rubio in the primary here on Tuesday to justify continuing his campaign into South Carolina.

“If Rubio beats him badly in New Hampshire, Jeb is toast,” said Senator Lindsey Graham, Republican of South Carolina, a Bush backer who spent most of Tuesday traveling the state with him. Mr. Graham added that Mr. Bush must either “tie Rubio or beat him” in this state.

That’s way off-message from the rest of Team Bush, which (momentarily) is insisting that they’ve got plenty of dough to go on to South Carolina and beyond. The whole value of Graham’s endorsement, in fact, is that he has some pull in South Carolina; if Graham’s now telling Jeb to hit the bricks before SC votes if he can’t beat Rubio in New Hampshire, presumably it means Graham is open to consolidating behind Rubio soon to stop Cruz and Trump. And he wouldn’t be the only Bush supporter who is.

All we’re looking at here is Senate endorsements, though. Could be that Rubio is ready to roll out Nikki Haley or Scott Walker, who talked openly about forming a ticket with Rubio back when he was still in the race last year. And of course there’s always the chance that Mitt Romney will decide that he has no choice but to knife Christie and Jeb Bush by diving in on Rubio’s behalf to help center-right voters consolidate as quickly as possible now. If Rubio’s building to something before Tuesday, that’s who I’d bet on — Walker or Romney, both friends of Chris Christie but both astute enough to know that Christie’s going nowhere at this point, no matter how many times he calls Rubio “the boy in the bubble.” Although a Twitter pal asks a good question: If Rubio had Walker in the bag, wouldn’t he have rolled him out before Iowa, Walker’s strongest early state?

For your viewing pleasure, here’s Jeb Bush in his final days as a candidate asking an audience in New Hampshire to clap.

Here's Jeb Bush asking an audience in New Hampshire to "please clap": https://t.co/xiaOYIufy0 @shiftmsnbc pic.twitter.com/x0Uc5Fl8gr

— NTA by Mic (@NavigatingTrump) February 3, 2016

Update: And here’s Toomey making it official.

Update: Here’s another (former) senator from Pennsylvania onboard with Team Rubio. Your move, Huck.

Rick Santorum tells Greta he is endorsing Marco Rubio as a "born leader"

— HowardKurtz (@HowardKurtz) February 4, 2016