Donald Trump has his first big-name primary challenger: former Massachusetts governor and 2016 Libertarian Party veep nominee, William Weld.
I'm from Massachusetts, and I think it's hard to overstate how beloved Bill Weld is in Massachusetts. He defeated an unpopular Democrat who was so socially conservative that towns like Amherst and Provincetown went red. He balanced the Massachusetts budget and created a surplus. (And I mean he actually balanced it, not Mitt Romney "I line-item-vetoed" some random stuff that I knew the legislature would override me on, so if the budget isn't balanced blame them" balanced." He stood up to the legendary bigotry of Jesse Helms, which cost him an ambassadorship.
So I should love the chance to vote for Bill Weld in the GOP primary. But, as Nathaniel Rakich of 538 points out, even in Massachusetts, where moderate Republicans still could traditionally do well, Weld's protege is struggling against a state party captured by Trump, and as a pro-choice, former Libertarian veep candidate who is to the left of me on guns and pretty much endorsed Clinton, it's not clear what constituency in the GOP there is for him to capture. Even in 2006, with the backlash against W's style of "big government conservatism" Weld couldn't secure the gubernatorial nomination of neighboring New York.
There are, as I see it, four states were Weld plausibly could win a primary, all in New England, and I don't see him definitely winning any of them. Weld is well-known in Massachusetts, the other five New England states, and New York, and I think that there his name recognition pretty much ends. Massachusetts, Vermont, and New Hampshire all have large numbers of libetarian-ish independent voters eligible to vote in their semi-open or open primaries, so he could win anywhere from one to none of these states and I just nod and say "yeah, that's about right." Rhode Island seems like a less-likely pickup because libertarianish voters make up a smaller chunk of the electorate, but he's also a moderate and moderate candidates play well there, so I expect he won't win RI but wouldn't be shocked if he did.
Maine has a semi-open primary, but Maine is different from other New England states. Maine Republicans are both more numerous than in MA, VT, or RI, and much more socially conservative and economically "liberal" than Weld is, perfect for Trump and a poor fit for Weld. Hell, Maine Republicans embraced Paul LePage, who was Trump before Trump. It's not outside the realm of possibility he wins the Maine primary on the strength of independent voters, and I wouldn't quite be shocked if he did, but I'd be very surprised.
Connecticut and New York both have closed primaries, which means I think Weld has no chance of beating Trump in those states, especially with New York being Trump's base and Connecticut being where all the Wall Street bakers who work in NYC live. Weld winning either of these states, Weld even coming close to winning (say breaking 40%), would shock me, and yet probably not mean much beyond "Trump's support among suburban business Republicans in blue states he wasn't going to win anyways is now completely dead."
Trump's bigotry combined with the changes to SALT deductions could result in a scenario where Weld scores maybe 30% in the primary in New York and CT, which would be a minor embarrassment to Trump that he would play off as the greatest victory ever. In states hard-hit by the SALT changes, places like California, Washington, and Maryland, if there is no other major primary challenger and those states hold primaries, I could see Weld racking up 20-30% as the "not Trump" candidate, but I suspect at least one other not-Trump candidate who is more conservative and has national name recognition will steal that mantle, hence the title of this thread.
On top of this, Republican officials have the option of not holding primaries if there's a sitting president of their party, and just giving their state's delegates to Trump, which a lot of state parties are considering doing.
I will say that in the fantasyland scenario where Weld somehow snatches the GOP nomination from Trump, while I would continue to vote for Democrats for Congress because I am not forgiving the GOP for their embrace of Trump and especially for Kavanaugh, there is no candidate running for or even likely to run for the Democratic nomination I'd vote for for president in preference to Bill Weld.