There is a reasonable chance that Mr. Trump will get a majority of delegates on the first ballot at the Republican convention. It is generally assumed that this guarantees him the nomination: majority election is interwoven into our political fabric so tightly we assume that it is a fact of nature.
Well, it isn’t. It’s a rule of each party: in the case of the Republican Party, it’s Rule 40(d) of the Republican Party Rules. And these rules can be, and are, modified at each convention. There’s been lots of speculation that the RNC would modify Rule 40(b) to permit individuals that hadn’t won eight states to be placed in nomination. If they can do that, they can raise the threshold for nomination.
So how much should they raise it? One reasonable target is 2/3 of the delegates, or, this year, 1650 delegates. This was actually the standard for the Democratic Party from 1832-1936, when it was abolished at the time of the unanimous renomination of FDR. Interestingly, the rule had been suspended in 1836 and 1840, only to be reinstituted to prevent the renomination of President Martin van Buren. It’s worth noting that every Republican nominee since 1976 easily passed the 2/3 threshold, and in the postwar years only Eisenhower (1952), Richard Nixon (1968), and Gerald Ford (1976) won on the first ballot with less than 2/3 of the vote.
It is still mathematically possible for Mr. Trump to win 2/3 of the delegates; only 734 delegates to date are committed against him, and it would take 825 to keep him from a 2/3 majority. If he runs the table on the remaining primaries, he could do it.
Now, should the GOP stop him this way? Of course they should: Mr. Trump is not an acceptable candidate for President of the United States, or the leadership of any major Democracy. He has disqualified himself through his unceasing threats of lawlessness and violence. This has nothing to do with his policies or ideology, or, frankly, lack of same: it has everything to do with his behavior. His behavior and rhetoric would have had him censured or expelled from any legislative body; recused from any bench, and likely impeached; fired for cause as an executive with any public corporation. Should a political party have lower standards than that? Of course not; the simplest and most direct solution would be to just refuse to permit him to have his name placed before the convention. Failing that, at least require him to get a supermajority.