I was a Clinton supporter in 2016 but Sanders would have been fine with me. I did some fairly extensive oppo research on Sanders to use in Facebook arguments with friends who opposed Clinton. The most serious problems I found (simple enough that a Trumper could use them) were Sanders’ support for leftist / anti American regimes; a lewd (some might say misogynist) short story Sanders published in his youth; the violence (verbal or physical) of a tiny fraction of his supporters; Saunders’ position on Palestinian statehood (which could be construed as anti Israel); questionable funding / loan for Jane Sanders’ college. There is some history of non-progressive viewpoints that could be used by Trumpers to sow discord among Democratic voters (Sanders’ position on the Palestinian question could be construed as not anti-Israel enough); support for the F-15 fighter jet; support for pro-life downstream candidates; various stances / remarks that aren’t feminist enough; lack of support from the African-American community. And of course there are the various positions Sanders has taken since 2016 that could be used against him with a moderate / conservative audience (on open borders, etc.).

But here’s the final take: Oppo researchers can find similar problems with any Democratic candidate. I seriously doubt that there is some damning secret in Sanders’ closet that doesn’t have a parallel in everyone else’s closet. Clinton was “vetted” for decades before 2016; that didn’t help her any — the Republican right used new “scandals” like email, and ramped up the old scandals as if they were new.

If you like Sanders, support him as you see fit. There’s nothing to be gained from acting out of fear of dirty tactics from Republican shills. They’re already using these tactics with Biden and Buttigieg, and they’ll use dishonest smears anytime they can.