What could possibly explain this? Not just Carlson, but Gaetz, Boebert and others. Are they still pissed off at Zelenskyy because of the role he played in humiliating Trump when the latter tried to blackmail him? Is it a residual effect from the Trumpian open admiration for autocrats and dictators? Or is it just a reflexive response to Biden’s adept support of Ukraine in the political, military and diplomatic front. I.e., anything Biden is for, especially if he is successful at it, MUST be evil.

Carlson's nightly pro Putin rants have a big influence on his several million highly gullible viewers. That is a big part of this, but the other things you mention are clearly big factors too.

If I were a partisan demagogue, I would welcome any issue I could agree with my opponents on, if for no other reason than to establish my fairness and impartiality, and to deflect charges of “partisanship”.

Me too. I have pointed out to conservative friends that I supported Trump's return to the moon program, the space force, moving the US embassy to Jerusalem, and a couple other things. They cannot support a single thing Biden does regardless even if it is a continuation of Trump policies!

Then again, perhaps this is just an exploratory foray, to see if they can find some new public opposition to the Democrats that they can exploit among their knuckledragging constituency.

Possibly.

Or perhaps, maybe Russian social engineers still have leverage in the American Conservative movement and are calling in some of their markers.

Absofuckinglutely.