It's about the delegates now. By that measure, Tuesday was a great night for Hillary Clinton, and a mixed one for Donald Trump.
On the Democratic side, it's pretty straightforward. Clinton wound up winning a squeaker in Illinois and leads narrowly in Missouri, but solid wins in North Carolina and Ohio and a blowout in Florida expanded her already unbeatable lead in delegates.
For his part, Trump won Florida and all 99 of its delegates. He also won in North Carolina, a proportional allocation state, and in Illinois, where the winner takes most delegates. The statewide count was close in Missouri between Trump and Ted Cruz.
But John Kasich won in Ohio, taking all 66 delegates, and will presumably stay in the race. Some people think his continued presence hurts Trump: That is, he'll pick up some more delegates, doing well in states where Cruz can't, and therefore help prevent Trump from wrapping up the 1,237 delegates he needs for a majority at the convention. But I lean toward the view that Trump is better off if his opposition is split between the Ohio governor and the Texas senator.