How? you ask. Remarkably easy: all they would've had to do is keep 5 or 6 other candidates in the race. Trump got more votes than any other candidate in almost every Republican primary, but while there were a bunch of other candidates in the race, he was still getting far less than 50%. That's back when he was threatening that there would be riots in Cleveland if he wasn't named the nominee, remember? Back when it didn't look at all certain that he'd come to the convention with 50% of the delegates. He claimed he had a right to the nomination whether he had 50% or not.
And of course, he was wrong about that. The way both the Democratic and Republican conventions have worked, for a long, long time, maybe for as long as there have been Republican and Democratic conventions, I don't know, but definitely for a long time, is that if no one gets 50% on the first ballot, you vote again and again until someone has 50%. and after the first ballot, no delegates are pledged anymore.
All the Republicans had to do, back during the primaries, was keep Cruz and Rubio and Kasich and a couple more people in the race against Trump, and get together and decide who was going to win after the first ballet.
Oh, that doesn't sound simple to you? Compare it to trying to nominate someone else now, with a clear majority of delegates committed to Trump. My way would've been easier. And there was no reason for them to be intimidated by Trump's threats of riots: on the contrary, that's the sort of statement they easily could have used against Trump. Well, looks easy when Hillary Clinton and Elizabeth Warren and John Lewis and Debbie Wasserman-Schultz and Barack Obama and Barney Frank and Nancy Pelosi and Howard Dean do it.
I think that the effort to stop Trump at the convention, when he has a big majority of delegates, will fail spectacularly, and sow acrimony and discord among Republicans generally, and leave Trump an even weaker candidate than he is now. So, as a Democrat, I'm all for it. I suspect that some of the Stop Trump Republicans have given up a while ago on winning the Presidential election, and figure that the best they can do for their party is to put as much distance between it and Trump as they can. Still, it's their party that put Trump in the the position he's in now. To some extent, all Republicans have to own that at least a little. And I think that shows that their party is in deep trouble.