Have I become too pessimistic about stopping Trump?

Maybe. Consider some math, brought to us by Rich Lowry (via NBC’s first read):

The delegate math: How a contested convention could happen: Want to see how the Republican Party is likely to wind up with either 1) Donald Trump as the clear favorite to win the GOP presidential nomination, or 2) a contested convention? Well, here’s the delegate math:

Trump holds an 88-delegate lead over his closest competitor, Ted Cruz, according to NBC News’ delegate count.
Donald Trump 325 (45 percent of all delegates)
Ted Cruz 237 (33 percent)
Marco Rubio 117 (16 percent)
John Kasich 27 (4 percent)

If you assume that the upcoming proportional contests from March 5-12 (including Kansas, Kentucky, Louisiana, Michigan and Mississippi) break down how the previous 15 races did, you end up with these delegate totals:
Trump 485
Cruz 355
Rubio 174
Kasich 41

Then on March 15, if Trump captures the winner-take-all races of both Florida (99 delegates) and Ohio (66 delegates) and wins 45 percent of the rest of the delegates on that day, the totals become:
Trump 741
Cruz 422
Rubio 206
Kasich 49
In that case, Trump needs to win 50 percent of remaining delegates to get to the magic number of 1,237, which is more than doable with Rubio and Kasich likely dropping out after losing their home states.

But if, say, Trump wins Florida but loses Ohio to John Kasich, the numbers become: Trump 675
Cruz 422
Rubio 206
Kasich 115
In that case, Trump needs to win 57 percent of remaining delegates to get to 1,237 — still doable.

But if Rubio wins Florida and Kasich wins Ohio, the delegate totals are:
Trump 576
Cruz 422
Rubio 305
Kasich 115
In that case, Trump still holds the delegate lead. But he needs to win 66 percent of remaining delegates to get to 1,237 — and that could be a hard climb.

A scenario in which Trump goes into the convention as the leader in delegates but is blocked because the competition coalesces around another candidate is hardly ideal. It could result in Trump running as a third party candidate or, at minimum, many of his backers staying home in November.

That’s why, in addition to Trump losing in Florida or Ohio, it would be very nice to see him limp to the finish line. Denying him the nomination would still raise problems, but they might be less acute.

There are a lot moving parts here. Sort them all out and the race clearly looks like Trump’s to lose. But maybe I have been too pessimistic about the possibility of him losing.