Some of them are actually worried that Trump has become a pawn outright controlled by Putin. If true, this is arguably one of the gravest threats the United States has ever faced. It's far worse than when the Kremlin had moles like Kim Philby inside the highest levels of Britain's intelligence operations.
Putin isn't just a head of state. He's a former KGB spymaster. He has considerable experience working foreign pawns to do his bidding.
And Trump is easily manipulated. He can be controlled by flattery. He can be tricked into ignoring what doesn't flatter him. And he's pathologically unable to keep his mouth shut and has, more than once, blabbed highly sensitive intelligence to dangerous parties.
Trump is too emotionally unstable to actually be a Putin agent, but he's the perfect pawn. Vladimir Putin trained for decades to handle insecure people like Trump. And the fact that Republicans in Washington are busy looking the other way creates an ideal environment for Russian espionage.
Hopefully Trump can be removed before he does too much damage. Vice President Michael Pence would be a terrible President, but arguably he'd be wary of Russia and not so easily maniuplated.