The minute that polls opened in Georgia on March 1, Donald Trump was already winning the state's primary by 19 percentage points. The very second the first door of the first polling location swung open, Trump already had 116,000 votes in the bank, nearly a quarter of the total he would receive and enough votes, by itself, that if no one had voted for him that day he would still have come in third.
Georgia's use of early voting meant that Trump voters could go to the polls at any point from February 5 on to cast a vote. As exit polls reported by CNN indicate, people who made up their minds early in the race were much more likely to back Trump than voters who decided later.
That meant that Trump was able to bank a lot of votes early and that, even as late deciders decided against him, he had a large nest egg. Even as his opponents beat up on him in that last debate, Trump already had tens of thousands of votes that couldn't be taken back.
Most voters in Georgia voted on Election Day, but nearly 1-in-5 votes were early votes. Trump won 44 percent of the early votes, compared to 37.5 percent of those on March 1 itself.
He would have won regardless -- the final margin was 15.2 percentage points -- but he could have done much worse on Election Day and still won. Take 100,000 Election Day votes from Trump (more than a quarter of his total) and give them to Ted Cruz and Marco Rubio, and Trump still wins by 30,000 votes.
This is why Rubio's current position in Florida is so problematic.
The election is on March 15, so while Rubio trails in the polls, he has time to make up the gap (though he doesn't really seem to be).
But early voting in the state began on Monday in 17 counties, with others coming online this weekend. Those people are voting while Trump has a significant lead over Rubio, so it's safe to assume that the plurality of those votes will end up going to Trump.
A report from Politico indicates that Rubio isn't poised to bank votes of his own. Over the next 11 days, campaigns in the state should be pushing their supporters to early vote so that the campaigns have fewer people to target on the actual day of the vote. It's not clear that Rubio's team is well-positioned to do this. Bloomberg's Sasha Issenberg suggests that "[t]hroughout the year, the campaign had made only symbolic investments in field operations—enough to convince the press and local party figures that he was taking seriously grassroots interaction but not enough to dramatically shape the electorate through them."
Rubio almost certainly needs to win Florida in order to stay in the presidential race. His campaign has promised that he will win it.
At this moment, as you read this: He's probably already losing.