Let's start with South Carolina. Polling, for months, has suggested the race is Donald Trump's to lose. The battle then is for second and third, with Ted Cruz likely to occupy one of those slots and the "establishment" three — Rubio, Jeb Bush and John Kasich — battling for the other.

The key for Rubio in South Carolina is not to beat Trump; it's to beat the two other establishment candidates. If he can beat Cruz, so much the better. Haley's endorsement should help Rubio do the first part of that equation — for at least three reasons.

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1. She is immensely popular among Republicans in the state. A Winthrop University poll conducted in December 2015 showed her with an 81 percent approval rating among likely Republican primary voters. Among self-described tea party voters, Haley's approval rating was 84 percent. While no endorser conveys his or her popularity directly to an endorsee, Haley's decision to throw her lot in with Rubio will likely make some voters give him a closer look and others give him a second chance after a poor showing in the New Hampshire primary.

2. The endorsement will dominate headlines in the state. You can be certain that every newspaper in South Carolina will have this news on its homepage today and on its front page tomorrow. Add to that the massive amount of news coverage nationally that the endorsement will draw, and Rubio will have won at least one of the final three days leading up to Saturday's vote.

3. Undecided voters tend to be influenced heavily by momentum. It's human nature -- we like to be with the person who seems to be moving upward rather than the one slipping downward. (It's why there are SO many Golden State Warriors fans across the country these days.) Having someone as well known and popular in the state as Haley (not to mention Sen. Tim Scott and Rep. Trey Gowdy) behind Rubio will make him look to many people like the "hot" candidate in the race. Being with someone who is starting to look like a growth stock will tip a bunch of people off the fence for Rubio -- and away from Bush or Kasich.

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Now, before I go any further, yes, Nikki Haley endorsed Mitt Romney in 2012. And, as you may remember, it was Newt Gingrich, not Romney who won the South Carolina primary that year. But, Haley is much more popular among Republicans today than she was then, and this is a very different race. South Carolina in 2016 is about the establishment finding their guy -- and Haley's voice matters, a lot, in that conversation.

Don't take it from me. Here's what Jeb, who badly wanted Haley's endorsement, told NBC's Peter Alexander on Tuesday about the governor: "She is the probably the most meaningful endorsement if there is, if she is going to give an endorsement it would be the most powerful meaningful one in the state." Oomph.

Assuming that Haley helps Rubio into a solid third or even second place in South Carolina, the senator emerges from the Palmetto State as the de facto establishment candidate in the GOP field -- the chosen one to take on Trump and Cruz throughout March. Bush will likely be pressured to leave the race, and while Kasich will almost certainly continue on until the Ohio primary on March 15, he won't challenge the top three's dominance.

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If that all comes to pass -- and it's the most likely scenario as of this moment -- then Haley will get credit for uniting the party behind what most establishment types believe is their strongest general election candidate. (Will she deserve that much credit? Probably not. But, if Rubio collapses somehow in the next few days, she will get the blame. So....)

And if Rubio winds up as the nominee, then you can be certain that Haley's endorsement of him in his time of need will not be forgotten. Haley was already at or close to the top of most vice-presidential lists, but if Rubio winds up as the party's standard-bearer, then Haley will be the runaway favorite to be his pick for the second-in-command slot.