Never Trump conservatives are fond of repeating it as if it were a mantra, but it is unwise to take such assertions at face value. It can be very difficult to distinguish what one firmly wants (a moderate and not a leftist in the White House, in the case of Never Trumpers) from what one actually believes.
This does appear to be a testable claim, however. There is some evidence that Trump is in fact salivating at the opportunity of running against Sanders. One merely has to consider who he has evidently trained most of his efforts at tearing down: Sanders’ chief rival in the polls, Biden. The whole Ukraine scandal was, after all, prompted by a desire on Trump’s part to smear Biden.
This theory that Trump actually wants to run against Sanders is bolstered by the fact (recently discussed here) of Bernie’s branding issues.
It is important to point out that Biden is hardly the slam-dunk candidate many centrist pundits seem to think he is, however. Rachael Bitecofer (one of the few analysts to correctly predict both the Trump win in 2016 and the size of the 2018 Democratic wave) has pointed out that Biden has some real risks, risks completely related to his political centrism, that are being overlooked by centrists. (I’ve pointed those risks out, too, but most people are likely to take a professional like Bitecofer more seriously.)
What to do about it all? It depends on who leads in the primary. If Biden is the clear leader, it’s worth supporting Bernie as a foil to Biden’s risky pro-Establishment proclivities. That probably won’t make Biden want to nominate Sanders as his VP,* but it will motivate Biden to tack left and embrace a few populist points, and hopefully nominate Warren to be his running mate.
Thankfully, given his front-runner status, Biden seems as if he might finally be getting it. That ad I just linked only mentions Trump in passing, talking mostly about what Biden says he will do if elected. That’s the sort of campaigning it took for the oppositions in Venezuela and Italy to finally have some measure of success against Chávez and Berlusconi, respectively.
* He’s never in a million years going to nominate Sanders to be his running mate. No Democrat would. After all, Sanders isn’t even a real Democrat. He’s always run as an independent or a socialist, and never been short on words with which to disrespect the Democratic Party. Yes, this is all for good reason (the Democrats have earned every bit of disrespect they receive from the left), but it’s also not precisely going to endear him to those whom he disrespected. Trust me, he won’t be nominated to be veep.