WASHINGTON — A bipartisan trio of senators is joining the legislative effort to boost legal protections for Americans who tip off authorities about suspicious terrorist-related activities.

Senators Lieberman, Collins, and Kyl on Friday said they were introducing a bill that would give civil immunity to whistleblowers who report possible threats to the transportation system. The legislation mirrors a measure sponsored in the House by Rep. Peter King of Long Island. That amendment, introduced as a procedural “motion to recommit,” passed in late March despite opposition by some Democrats, who argued on civil liberties grounds that the measure could encourage the unfair targeting of people based on their ethnicity or appearance.

The push gained new life following the arrest of six suspects charged in a plot to attack Fort Dix in New Jersey, which came after a video store worker reported suspicious behavior to the authorities.

“The Fort Dix incident reminds us that ordinary people can and do play an important role in the security of our communities,” Mr. Lieberman, an independent of Connecticut who caucuses with the Democrats, said in a statement.

Mr. Lieberman is the chairman of the Senate Committee on Homeland Security; Ms. Collins is a Republican of Maine, and Mr. Kyl is a Republican of Arizona.

Mr. King, a Republican, originally authored what is known as the “John Doe” amendment in response to a lawsuit filed by the Council on American-Islamic Relations on behalf of six imams who were removed from a USAirways flight in November.