US Democratic legislators who voted for President Barack Obama's historic healthcare reform have received death threats and had offices vandalised.
Democratic House Majority Leader Steny Hoyer said more than 10 of them had reported incidents since Sunday's vote
Some also received threatening phone calls and have raised concerns about their personal security.
Police and the FBI are investigating the incidents which Mr Hoyer described as "very serious".
Mr Hoyer said people had shouted that Democratic lawmakers should be put on firing lines and posters have appeared with the faces of lawmakers in the cross hairs of a target.
The office of Democratic Representative Bart Stupak received a phone message from a male caller saying: "I hope you bleed..., get cancer and die."
Another incident involved a gas line being cut at the home of one legislator's brother in a case of mistaken identity after the wrong address was posted online.
Legislators who feel they are at risk will be "getting attention from the proper authorities", Mr Hoyer said.
House Republican leader John Boehner said that while many Americans are angry over the bill's passage, "violence and threats are unacceptable".