As we come down to the Final Four in the Republican Presidential race — Donald Trump, Ted Cruz, Marco Rubio, and John Kasich — one person seems to hold the key.
Who is that person? According to one way of thinking, that person is Marco Rubio. If Rubio continues to stay in the race, he splits the "non-Trump" vote with Ted Cruz. This means Ted Cruz will continue to get beat by Donald Trump — and Trump will gain the nomination.
If Rubio withdraws, now or after Florida, and if John Kasich does as well, then it becomes a two person race. So far, about 1/3 of voters favor Trump and 2/3 favor someone else. Some of Rubio's and some of Kasich's voters will go to Trump, but a number will probably go to Cruz. Now you have a true two person race. Will Trump still win? Perhaps he will.
But it seems highly probable that if Rubio stays in, the "non-Trump" vote will be split and Donald Trump will win.