It's over. Trump will be the Republican nominee for President.
Even if every undecided Republican voter broke against Trump and every supporter of other candidates went to a single alternative, Trump would still win the nomination.
The political reality of the contest to be the Republican nominee for President in 2024 is that Donald Trump is effectively the incumbent President. It is extremely difficult to unseat an incumbent President in the nomination contest, even when the incumbent has annoyed a significant part of the party's base, as Pat Buchanan found out in 1992. Buchanan did not win a single state in his challenge to George H. W. Bush, and I doubt any of Trump's opponents will win any states in 2024.
The primaries have not started, but Trump - who, again, is effectively the incumbent - has a commanding lead with more than 50% choosing him. This is not 2015/2016, where Trump won multiple early primaries with a plurality of the vote. If the non-Trump vote had coalesced around a single candidate by December 2015, Trump may have been defeated. Now, even if every undecided voter broke against Trump and every supporter of other candidates went to a single alternative, Trump would still win the nomination.
The candidate with the most to lose at this point is Ron DeSantis. (Or, as Trump calls him, "Rob DeSanctus.") He gains nothing from this point forward by staying in the race. He risks hurting himself for 2028, especially as Trump's attacks and personal mockery escalates. We are already seeing a growing number of "Only Trump" voters sour on DeSantis, and that could hurt his chances of being the Republican nominee five years from now. Ted Cruz, Trump's strongest opponent in 2016, never recovered from Trump's attacks and will never be President. DeSantis would be wise to drop out now before too much damage is done.
Other than Chris Christie and Mike Pence, none of the Republican candidates have articulated any reason they should be running at all. Just by virtue of the fact that they are running against Trump, the other Republican contenders are saying that Trump is not qualified to be the nominee or even to be President again. DeSantis has come the closest to making that argument, but even he has held back from making the case that he needs to make if he has any hope of wrestling the nomination away from Trump.
Can Trump win the 2024 general election? Because of demographics, any major-party candidate has a chance of becoming President. Biden is being hurt by the economy and inflation, and is personally unpopular in a way he was not three years ago. The COVID-19 voting rules made it easier to vote three years ago, and many of those "emergency" measures will not be in place a year from now.
Biden won the popular vote three years ago, but that was mostly due to running up the score in California. The election was decided by fewer than 43,000 votes across three states: Wisconsin, Arizona and Georgia. Had Trump turned out just 43,000 more votes, he would have been re-elected. Furthermore, Pennsylvania (which Trump won in 2016) was decided by fewer than 82,000 votes out of 6.83 million votes cast for the two major-party candidates. Even if one could make the case that another candidate is more electable, Trump's supporters in the primary do not believe that. It does not matter who is actually more electable unless you can change minds.
But Trump's primary weakness is that he is hated by Democrats, far more than any other Republican. There are people who will turn out to vote against Trump in 2024 who would not turn out if someone else is on the ballot. Think about this: Trump got more than 11 million more votes in 2020 than he did in 2016 (beating Hillary Clinton's 2016 popular vote total by more than 8 million votes) and he still lost. Republicans, who want a candidate who makes liberals angry, need to recognize the harm they are doing to themselves and the country by needlessly antagonizing people. Trolling and taunting are poor substitutes for a solid policy agenda.
Trump is a bad candidate who has moved to the left on policy. In a perfect world, he would not be anywhere close to being the Republican nominee for the third consecutive election, much less elected President again. That, however, is where we are. Republicans who actually want to be President (and have a legitimate chance of winning in five years) need to seriously consider whether this race is a good investment of their time and money. Here is a spoiler: It is a bad investment.