Representative John A. Boehner of Ohio, the Republican leader, spoke out against violence but encouraged Americans to continue to find ways to counter the legislation. “I know many Americans are angry over this health care bill, and that Washington Democrats just aren’t listening,” Mr. Boehner said in a statement. “But, as I’ve said, violence and threats are unacceptable. That’s not the American way. We need to take that anger and channel it into positive change.”

Republicans also noted that their members have received threats in the past and that Senator Jim Bunning, Republican of Kentucky, was recently the target of abuse when he blocked added unemployment benefits.

Still, the dark and personal tone of the final stages of the health fight could complicate Republican efforts to maintain their attacks on the legislation if they are seen as inciting an undue level of outrage and, conversely, could bolster Democrats if opponents of the measure are seen as breaching the boundaries of civility.

The reports of threats, coming after a tense weekend when protesters hurled racial and homophobic slurs at Democrats and spit on one congressman, left many Democrats shaken.

Paul Bresson, a spokesman for the F.B.I., said the bureau was aware of several threats against members of Congress and was investigating them jointly with the Capitol Police, which handles security for Congress, and local law enforcement agencies. He declined to say how many cases there were or to describe them in detail. “We are looking at them and taking them very seriously,” Mr. Bresson said.

Democrats also raised questions about some of the imagery and phrases being employed by Republicans against the Democratic architects and backers of the measure, noting that a Republican National Committee Web site urging supporters to fire Ms. Pelosi has her surrounded by flames. A Facebook page of Sarah Palin singling out Democratic members for defeat because of their votes defines their districts by the crosshairs of a weapon’s sight.