Several members have faced death threats and harassing phone calls while others have experienced vandalism at district offices or their private homes.
In one case, a gas line was cut outside the Virginia home of Rep. Tom Perriello's brother after a conservative activist posted the address online, mistakenly thinking it was the congressman's house. The Federal Bureau of Investigation is investigating.
"All threats and incidents directed against members of Congress are taken seriously and are being investigated …to identify and bring to justice those responsible," said FBI spokesman Paul Bresson.
This behavior "undermines our democracy and undermines the safety of individuals," House Majority Leader Steny Hoyer, D-Md., said following a closed-door briefing by law enforcement officials this afternoon.
"Any member who feels themself at risk is getting attention from the proper authorities," Hoyer said. Normally only members of party leadership have enhanced security.
Republican House Minority Leader John Boehner, R-Ohio, denounced the threats and acts of violence, saying it's "not the American way."
"I know many Americans are angry over this health care bill, and that Washington Democrats just aren't listening. But, as I've said, violence and threats are unacceptable," he said in a statement.
Still, Hoyer and other party leaders have been increasingly critical of what they see as dangerous rhetoric by Republicans and tea party activists.
Hoyer today obliquely referenced the online campaign by the Republican National Committee to "Fire Pelosi" and an e-mail from Sarah Palin's PAC that depicted targets/crosshairs over the congressional districts of some Democrats who voted for the health care bill.
"When people start talking in the rhetoric of putting people on firing lines, that if they don't do something, they will have physical harm done to them, that other rhetoric of that type, or they put a target on their faces with crosshairs, that activity ought to be unacceptable in our democracy," he said.
RNC Fires Back at Dems' Charges of Threatening Rhetoric
Republican National Committee Communications Director Doug Heye responded to Hoyer, saying it's Americans who will face higher taxes and health insurance premiums under the new health care law that are "on the firing line every day."
"Meanwhile, when Sen. Jim Bunning received death threats this month, Democrats didn't say a word. When Gov. Mitt Romney was assaulted on an airplane last month, Democrats [never] said a word. And when Gov. Sarah Palin is treated like a pinata every day, Democrats and a willing media give the polite golf clap."
ABC News' Jason Ryan and Jonathan Karl contributed to this report.