This article made the rounds back in '19:
https://www.policybear.org/post/are-there-dual-citizens-in-congress
Short Answer: Nobody knows, but prolly some.
The real answer is nobody knows since there's no law/rule saying members of Congress must disclose it.
And, imo, who financially backs or blackmails (as Q has suggested) the Congresscritters is way moar important than any dual citizenship/passport info about them.