WASHINGTON — A provision aimed at providing whistleblower protection to Americans who report suspicious activity has advanced on Capitol Hill despite opposition from some Democrats, as top Republicans announced an agreement last night.

The House Republican leader, Rep. John Boehner of Ohio, and Rep. Peter King, a Republican of Long Island, said last night that they had reached agreement to include what has become known as the “John Doe” amendment in the final version of a major homeland security bill to implement the recommendations of the commission established to investigate the terrorist attacks of September 11, 2001.

The bill is currently in a House-Senate conference committee, and a deal will be announced today.

The homeland security legislation mandates tighter inspections on cargo coming into America on passenger planes and ships, and it could help direct more anti-terror funds to New York by adjusting the funding formulas in favor of high-risk areas.

Disagreement over the “John Doe” amendment had been the last sticking point of the broader legislation, which the Democrats had pegged as one of their top priorities in taking control of Congress this year.

Mr. King first introduced the amendment earlier this year after the Council on American-Islamic Relations filed a lawsuit on behalf of six imams who were removed from a US Airways flight in November over allegations of suspicious activity.

The measure would offer civil immunity protections to individuals who tip off the police to such activity.

The amendment won House approval in March and a majority of senators, including Senators Schumer and Clinton of New York, supported a similar measure sponsored by Senator Collins, a Republican of Maine, when she tried to attach it to an education bill last week. But it fell three votes shy of the 60 it needed to pass.

Some Democrats had raised civil liberties concerns, saying the legislation would lead to unfair targeting of people based on their appearance, religion, or ethnicity.

The measure has gained momentum, however, amid the recent reports of terrorist plots, including one to attack Fort Dix in New Jersey in May and another, more recently, to blow up fuel tanks at John F. Kennedy International Airport in New York.

A leading Republican presidential candidate, Mayor Giuliani, is also weighing in. In a statement yesterday, he criticized Democrats who opposed it, saying they “just don’t get the terrorists’ war on us.”

He also praised Mr. King, who has endorsed his candidacy as someone who puts “national security first and political correctness run amok second.”

A potential Republican candidate, Newt Gingrich, also vouched for the bill this week, saying Monday that if Democrats succeed in blocking it, it would reflect a society with “suicidal tendencies.”

The final disagreement centered on the language of the bill. The Democratic chairman of the House Homeland Security Committee, Rep. Bennie Thompson of Mississippi, supported the provision but wanted to limit the immunity protection to those who report activity that is specifically terrorist-related.

Mr. King yesterday rejected such a limitation, saying it would “negate the whole bill.”

“It totally undermines the whole amendment because how is a private citizen able to make a decision about whether or not suspicious activity they see is going to be for a terrorist crime or an ordinary crime?” he said in an interview yesterday afternoon. “If they see somebody buying explosives, how do they know it’s going to be for terrorist purposes, not just an ordinary crime?”

Mr. King said Democrats such as Mr. Thompson and the House speaker, Rep. Nancy Pelosi, were facing pressure from the liberal wing of the party “who see any action that’s quote unquote tough on terrorism as being pro-Bush or pro-Republican.”

He said he would not endorse the homeland security bill unless it included a strong immunity provision.

An aide to Mr. Thompson said that while the congressman supported the measure, the language as drafted “was too broad.”

A spokeswoman for the chairman of the Senate Homeland Security Committee, Senator Lieberman, an independent of Connecticut, vowed that an immunity protection would make it into the final bill.

Mr. Lieberman, who caucuses with the Democrats but is conservative on military issues, was an initial sponsor of the proposal in the Senate.