He trails Mr. Trump in every survey, and often by a wide margin, with the latest polls showing him down by nearly 20 percentage points.
Florida's 99 delegates are enough to cover an eighth of those Mr. Trump needs to reach the majority count, but there is a silver lining for Mr. Trump's opponents: Mr. Rubio's decline, and the possibility that he could withdraw after Florida, is good news for them elsewhere.
Mr. Cruz has picked up a lot of Mr. Rubio's former supporters in firmly red states like Louisiana and Mississippi. Mr. Cruz lost states like these by a wide margin on Super Tuesday, but he nearly won in states where Mr. Rubio's support was reduced to the single digits. The same phenomenon has given Mr. Cruz a chance to squeak out wins in Missouri and North Carolina.
Farther north, Mr. Rubio's weakness helps Mr. Cruz and Mr. Kasich, but perhaps not by enough to allow either to overtake Mr. Trump. That seemed to happen in Michigan, where Mr. Trump won a much smaller share of the vote than he did in Louisiana or Mississippi but, because his opposition was more divided, still won by a larger margin.
The same phenomenon might help Mr. Trump win Illinois.
Sanders in the Midwest
Mr. Sanders can prove his upset victory in Michigan last week was not a fluke.
He has a real chance of winning in Ohio, Missouri and Illinois, where the polls show a much tighter race than they did before Michigan. Like Michigan, the three states might be particularly receptive to his message on trade, and they are less diverse than the nation.
A few more wins in the Midwest would strengthen the case for Mr. Sanders to stay in the race and compete in the delegate-rich blue states that dominate the final two months of the primary season.
But the challenge for Mr. Sanders is not simply to win in the region, it is to win big. Hillary Clinton could win Florida and North Carolina by more than 20 percentage points, making her likely to add to her big pledged delegate lead. Narrow victories by Mr. Sanders will not do much to cut into Mrs. Clinton's growing edge.
One advantage for Mr. Sanders is that the Midwestern states hold open primaries, where voters of any political affiliation can cast their ballots.
Mr. Sanders tends to fare best among independents, especially because younger voters are likely to identify that way. Exit polls in Michigan found that Mrs. Clinton won self-identified Democrats by 18 percentage points, but she lost the state nonetheless because of a 43-point disadvantage among independents.
There is a flip side to open primaries: Democrats can vote in the Republican contest. That might help Mr. Sanders, too. It has been argued that Mrs. Clinton was hurt because some of her supporters voted to stop Mr. Trump in Michigan. Self-described Democrats represented 7 percent of the Michigan Republican primary electorate, according to the exit polls.