Donald Trump speaks during a campaign stop in Tampa, Florida. (REUTERS/Mike Carlson)

There hadn't been a good poll in Florida for a while, until Thursday morning. There was no data to suggest how Jeb Bush's withdrawal might affect voting in that state, so Donald Trump's big advantage there seemed as though it might be misleading. After all, one would assume that Bush's traditional-Republican support in his home state would likely shift to the other traditional-Republican Floridian, Marco Rubio.

Quinnipiac University has a new poll out. If the election were held today, and not on Mar. 15 as it is, this would be the result:

If you are Rubio, there is no way of looking at this besides that it is an unmitigated disaster Trump leads easily with almost every demographic in the poll. Rubio only leads -- and does so narrowly -- with college-educated Floridians, a group that was half the GOP electorate in 2012. Trump leads among those who have no degree by a 2-to-1 margin. That was the other half of the voters.

There are margins of error at play, and we're still 20 days out and so on, but, again: disaster. About a fifth of the Republicans in Florida, 21 percent, say they'd never back Donald Trump. The percentage saying that about Rubio? Seventeen.

This poll also gives the lie to the newly popular idea that if John Kasich were to drop out, Rubio's path to the nomination would be cleared. If Kasich had dropped out before Nevada, and Rubio had gained all that support, Donald Trump would have won by 18 points instead of 22. If Kasich dropped out before Florida and every single one of his votes went to Rubio, Rubio would lose Florida by 9 points. If Ted Cruz and Ben Carson also dropped out, Rubio would need to pick up nearly two-thirds of those votes to beat Trump -- in his home state.

The reason we are talking about Kasich having to drop out is that Kasich doesn't have much support. So re-allocating even all of that "establishment" vote to Marco Rubio doesn't add much value to his candidacy.

Allocating all of Kasich's vote is faulty in its own right. NBC's most recent national poll with SurveyMonkey explored the second choices of supporters of each remaining Republican. A quarter of Bush supporters didn't know where they'd go. Eleven percent went to Trump, 19 to Rubio. A quarter of Kasich supporters said they'd go to Rubio -- but 16 percent said they'd back Trump. A fifth of Carson backers would go to Trump, as would a quarter of Cruz supporters. These numbers vary by state, and so on, but the point is obvious: Voters are not moving between candidates as a bloc.

That's hugely advantageous to the guy who's already in the lead. He doesn't need to add as many supporters from his opponents as they do. FiveThirtyEight's Nate Silver noted on Twitter on Thursday night that Trump's base of support is very, very solid -- unlike many of his competitors -- and that he likely had room to grow.

People who are losing elections like to argue that anyone who isn't voting for their opponent is a vote against their opponent. That's nonsense and spin. Trump's 33 percent in the national polling average doesn't mean that 67 percent will vote against him, no matter what -- clearly, since Trump has beaten 33 percent in two of the four contests so far. The core group of anyone-but-Trump voters are that 21 percent in Florida (and higher percentage elsewhere).

As Silver notes, the middle ground for Trump is smaller -- but it's not non-existent. Here's what it looks like from another, national Quinnipiac poll.

In order for Trump to lose, it will take far more than Kasich dropping out. It will take the sort of dramatic shift in the electorate that could cause Donald Trump's 44 percent share in Florida to collapse -- or at least crumble -- by Mar. 15.

This is not impossible; Trump's support has consistently fallen as primary and caucus days approach (both in polling averages and in the voter choices of people who decided late, according to exit polls).

But Trump also still keeps winning.

Donald Trump cruised to victory in Nevada, building a broad coalition that left his two rivals trailing far behind heading into Super Tuesday. The Washington Post's David Weigel talks about what this means for Trump 's chances of securing the GOP nomination. (Alice Li/The Washington Post)