|Sen. Lamar Alexander (R-TN)|
It's worth noting right now that this is possible, of course. Most people have spent months hearing things like 1237 and bound delegates, but it must be pointed out that those are only the rules under the current guidelines for the convention, and each convention technically has its own rules in this regard. The convention rules have not been officially set, and will be in the coming weeks, and Trump's lack of political position in the Republican Party (which goes even further downhill as the weeks continue) could come into play if someone really wanted to mess with him at the convention. The reality is that while the delegates have been chosen, the RNC could change the rules to make them unbound on the floor, or even create rules that would make it more difficult for Trump to win (requiring a majority of delegates from a number of states he'd be unlikely to attain, or mandating the nominee being a sitting or former holder of elected office). The reason they can do this is that it's often forgotten but true that the Republican Party (along with all other political parties) is technically a private entity. It exists as a framework for getting people elected to office, but it isn't required by law to have a specific set of rules to govern at its convention. Trump could, in theory, be relatively easily dismissed by the Republican Party at the convention.
So why wouldn't they do this, considering his horrendous polling numbers (seriously-the only time I've ever seen numbers like 70% disapproval from someone nationally is GW Bush during the financial crisis in late 2008) and the fact that he's going to be a drag on the rest of the ticket? Well, there's pragmatism and then there's strategy. Pragmatically, Donald Trump won their ticket under the rules that have long been established by the party. He won 1237 delegates and the correct number of states, which is what the party mandated of their nominee prior to the convention. For all of the talk of the contested convention and whether or not it would be fair to throw out Trump, Trump did what was required of him under the rules. There is a level of fairness and validity the party must maintain in order to maintain the public's trust. While it is a private entity, like all private entities it is entirely dependent on its customer-base to continue existing. When the Republican voters chose Trump as their nominee and he achieved the agreed-upon rules, there was a covenant with the voters that he was the nominee, for better or for worse. A plurality of the party would not trust the GOP in the future if they were to overturn the will of their constituents like that, and that could hold not just for November (where even a fraction of Trump's supporters staying home would not only give Hillary Clinton the White House, but take away the Senate from the Republicans), but it could make the entire primary process in the future look like a sham. There's a distinct difference between what is "technically right" and what actually appears fair. Overturning Trump's victory when he clearly and unmistakably earned it could tarnish the Republican Party for years, perhaps even causing a group of the party to permanently form a third party that would eat into their support for a generation.
You could argue, though, that there's enough time between the convention and November that people wouldn't care anymore. I'm always shocked in an election season how quickly people seem to forget about the losers. For example, by November, not a single person except for a handful of Twitter eggs are going to be talking about Bernie Sanders. We will be so far removed from the election that it won't matter how much of a run he gave Clinton-he'll just become a footnote that political scientists bring up for the next fifty years or so (and likely a powerful man on the hill if he gets a gavel). Bernie Bros, for the most part, will have moved into the Clinton camp or perhaps the Trump camp, but Sanders himself won't be a factor. The same could theoretically be true for Trump. It's hard to imagine Trump going away, but the media is about covering someone who is flashy, but more importantly flashy and powerful. If the simple idea was to just cover someone outlandish, Sarah Palin would have gotten just as much press over the past eight years as Hillary Clinton, but she didn't. She only received it while she was still considering a run for the White House, and the same has to be said for Trump. His supporters might still show up, but even that wanes when there isn't some sort of power associated with your speeches. The election and voters will focus on who is an actual candidate, and if Trump wasn't the nominee, he would get little to no news in this situation. One can argue that this might change with someone as outlandish as Trump, and indeed I do think overturning the will of the voters could be a game-changer, but past precedent shows that average people have ridiculously short memories when it comes to politics-it could be achievable without killing the ticket in November in theory.
The larger problem for the GOP, then, is that a contested convention, particularly one where you're throwing out the will of your party, would leave a treasure-trove of unknowns. The winner of the primary is anyone who gets 1237 votes. Right now, there are 1441 delegates in Trump's corner. While some of those are RNC officials that are being forced to stick around for Trump because he's the presumptive nominee and likely would bolt if they had an option, it's not inconceivable that the vast majority are actual supporters or would feel duty-bound, regardless of the rules, to support the man that won their state or territory. The worst possible situation for the GOP would be for them to contest a convention that feels finished and then end up with Trump still taking the nomination. That would have damaged the party (Trump supporters would hate the likes of Mitch McConnell and Paul Ryan), but left them with the same problem, except one who is famously prone for vengeance and surely would target the moderates who would have orchestrated such a coup, likely costing the Republicans in a number of congressional races.
After all, the eventual nominee in this situation has to get to 1237 votes, and it's no guarantee any other Republican could do so in such a situation, and swiftness would be imperative here. No Republican, pragmatically, could try and win the nomination without Trump sicking his attack dogs on them mercilessly in the process, so you'd have to essentially find a name at the convention. People like Ted Cruz, John Kasich, Marco Rubio, and Jeb Bush would have their names batted around, but they all already lost to Trump already and Trump supporters in particular may be vicious to the idea of one of the guys that Trump beat getting the nomination instead. Most of the rest of the party isn't famous enough (bringing up Nikki Haley or Brian Sandoval would leave them only weeks to introduce themselves to the country, and most other candidates haven't been properly vetted). Speaker Paul Ryan probably would be the best case scenario, but he's categorically said he won't run and it's doubtful, with a clear shot at 2020, he'd go into a hailstorm in 2016 and risk his reputation when four years of Hillary Clinton are so much better for his ambitions. Mitt Romney could be talked into it, and has led the anti-Trump charge, but he's also going to get called a turncoat in the process. The reality is that no Republican is obviously going to be the nominee this year in this situation, the bench is worn out-there is no GOP Joe Biden waiting in the wings "just in case." That's what's stopping this more than anything-the risk involved with taking down Trump is higher than the risk of keeping him. The latter just loses one election, the former could tear the party into two permanently.