A vote for Kasich or Rubio, in today's political environment, would be a vote for Hillary. If either candidate wins in Florida and/or Ohio -- and fights on afterwards -- it virtually guarantees a brokered convention. And a brokered convention would cause such havoc in the GOP that it would make Hillary's election much, much more likely.
Vote for Cruz; vote for Trump. But don't vote for Kasich or Rubio. Vote for a candidate who can win the convention on the first ballot.
Even in Ohio, the latest polling shows Ted Cruz surging to only six points behind Kasich and Trump, who are tied for first place. Those who want to vote for Kasich in order to defeat Trump in Ohio are misguided. Cruz has as good a shot as Kasich at beating Trump. And, if Kasich does win Ohio, he will stay in the race, constantly splitting the anti-Trump vote and giving Donald the nomination.
Trump has an easy time getting a plurality in a divided field, but has only been able to win a majority on very rare occasions. With Kasich in the race, Trump will win. With Kasich out, it is likely that Cruz will get the nomination.
Why not a brokered convention?
The Republican Party does not have the superstructure to resolve a convention deadlock. There are no more bosses. The state party leaders are largely impotent. The party lacks elder statesmen. The Bushes are compromised.
Romney is too self-interested (that's why he is campaigning for Kasich -- to cause a brokered convention that might nominate him).
A second or third or fourth ballot would lead to an endless deadlock that couldn't be broken easily. The fissure would rip the party apart and its impact would be to create such bitterness that it couldn't come together in time to beat Hillary.
A brokered convention would be a disaster. If Kasich and Rubio both lose and drop out of the race, the threat will have passed from our lips. But if they win and stay in the race, we are headed for the rocks.