Nevada, a state with a large Mormon minority, caucuses in a little more than 48 hours. If not now, when?

In a major development in the Republican presidential race, 2012 GOP nominee Mitt Romney will endorse Sen. Marco Rubio (R-Fla.) for president, The Huffington Post has learned from two Republican sources. Details of Romney’s endorsement were still being worked out as of Sunday morning. The sources said that the former Massachusetts governor had been eager to provide his backing to Rubio for days but had hesitated, due to his respect for former Florida Gov. Jeb Bush (R), who suspended his campaign on Saturday night following his dismal fourth-place showing in the South Carolina primary… Romney’s backing will provide Rubio with the highest-profile endorsement of the 2016 race and is the clearest signal yet that the party’s establishment is ready to coalesce around the Florida senator as its last best chance to defeat GOP front-runner Donald Trump.

As soon as that HuffPo story broke this morning sources inside Team Marco leaped to shoot it down, including chief spokesman Alex Conant and finally, as you’re about to see, Rubio himself. But it makes all the sense in the world that Romney’s about to take the plunge. The new dynamics of the race are straightforward: With Jeb gone and Kasich not even contesting primaries beyond his own midwestern backyard, Rubio’s officially the only game in town for establishmentarians. Romney, who won Nevada twice thanks in part to support from Mormon Republicans, is an especially valuable get ahead of Tuesday’s caucus and now he doesn’t need to worry about stepping on any toes by supporting Rubio. He’s going to endorse eventually and the guy he’s going to endorse is Rubio. It’s a question of when, and “now” makes the most sense as an answer.

So why is Rubioworld aggressively tamping this down? I can think of two possibilities. One comes via Alex Burns and bodes ill for Rubio and for the donor class’s confidence in him to stop Trump:

@allahpundit in theory, the case would be: Trump on track to win a blowout in NV & endorsing now would be a waste — Alex Burns (@alexburnsNYT) February 21, 2016

That seems plausible. Anything can happen in a caucus, which rewards superior organization (right, Iowa?) and Rubio can claim some momentum from beating Cruz in SC, but the two polls taken in Nevada this month have Trump winning by 16 and 26 points. Now he’s got another big win under his belt. Hard to believe Rubio’s going to contend there for anything better than another distant second, in which case why would Romney want to hop aboard now? All he’s doing is setting himself up for a week of laughing insults from Trump after he wins NV about how little influence Romney really has. Better for Romney to bide his time and endorse Rubio before, say, Michigan, where he has a better chance of surprising Trump. The problem with that theory, though, is that it assumes establishmentarians still think there’s time to stop Trump. There isn’t, and Trump’s going to attack Romney no matter when he finally weighs in with his endorsement. If Romney really cares about beating Trump he’ll have to risk a bit of pride by doing what he can to boost Rubio ASAP. Besides, Rubio’s path to the nomination increasingly depends on the race being seen as a two-man contest between him and Trump. Even if Cruz refuses to quit, some conservative anti-Trumpers in Cruz’s coalition will defect to Rubio over time. Dealing Cruz a distant third-place finish in Nevada on Tuesday would help Rubio create the “me or Trump” perception among voters, and Romney can help deliver that.

Which brings us to the second possibility: The rollout of the Romney endorsement is being carefully orchestrated and it’s not supposed to leak today. A few hours ago, it seemed unclear why Team Marco would be so sensitive about the timing. Granted, people aren’t paying much attention to the news on Sunday, but there’d be no harm in someone teasing the Romney endorsement before the big announcement tomorrow. Within the past few hours, though, it’s begun to make more sense: Dean Heller just endorsed Rubio at around noon ET and Marcoworld probably wants to carve out some time for that news to resonate in Nevada. The more buzz there is around Romney, the more it’ll cannibalize the Heller news. (And the news of a Brian Sandoval endorsement, which is probably also forthcoming.) What Team Rubio really wants over the next few weeks, I think, is a drumbeat effect of endorsements from GOP office-holders to create a sense that the party really is uniting behind him. That’s less important to winning over Trump’s fans than it is to convincing Cruz’s more mainstream Republican supporters that most of the anti-Trump vote in the GOP is destined to end up with Rubio. Rolling out Heller today and Romney tomorrow would start that drumbeat. If Romney hasn’t endorsed Rubio within 24 hours from now, though, than something strange is going on and we’re back to Burns’s theory. In fact, Romney holding back through Nevada, where he could do some real good, would feel like a microcosm of the paralysis in challenging Trump that’s defined so much of the establishment throughout the campaign. You’re not going to stop this guy by waiting and hoping for something good to happen. Frankly, I’d guess that it’s already too late to stop him.

There’s one other theory that could explain Romney’s reluctance, one which Trump fans on social media really like — Rubio simply doesn’t want Romney’s endorsement. Why would anyone, in this populist climate, be eager to have the benediction of the head of the Republican establishment, who got blown out by Obama in 2012? I think that’s silly, though, for all kinds of reasons. For starters, Rubio is obviously now the “establishment” candidate in the race for better or worse, and that’s true whether Romney endorses him or not. If Marco’s going to wear the scarlet “E” anyway, he might as well enjoy the benefits of an endorsement that come with it, especially on Romney-friendly turf like Nevada. Beyond that, I’d bet cash money that a poll taken this week would find Romney’s favorable rating quite solid among Republicans generally. The last time multiple pollsters tested him on favorability was last January, when he was thinking of running again. Monmouth measured him at 55/30 among Republicans; WSJ/NBC had him over 50 percent as well. YouGov, implausibly, had him at 82/12 around the same time. I bet he’s easily north of 50 right now within the overall party, no doubt in part because he actually did win the nomination whereas all of his comrades towards the center are flailing against Trump, and quite possibly much higher in Nevada. And don’t forget, a lot of people who are showing up in those polls as disliking Romney are probably already committed to Trump by now. Among the remainder of the party, which Rubio and Cruz are fighting over, I’ll bet he’s very popular. He’s more upside than downside for Rubio.

But even if you’re unpersuaded by all that, we have evidence that Rubio’s working to land Romney’s support. If you believe CNN, Team Marco reached out to MItt for his endorsement before New Hampshire. Romney held off, supposedly because he wasn’t sure Rubio had the numbers to win there. That was a shrewd bet, as it turns out, if you think the most important thing to the Republican establishment is to protect Mitt Romney’s political capital. If you think the most important thing is to stop Trump, then maybe a daring endorsement by Romney after Rubio’s debate debacle would have helped Rubio hold onto a second-place finish in NH, which would have set him up to threaten Trump in South Carolina. Regardless, Rubio and his allies seem to want Romney’s backing and they presumably know what’s best for their strategy, which should be the end of the “Rubio doesn’t want Romney’s support” theory. If Mitt doesn’t sign on today, I bet he does tomorrow. If not, it means even the GOP brain trust still isn’t taking the threat from Trump as seriously as they should.