The Irish republican lobby, using techniques developed by U.S. political parties, is mounting a campaign to block a revamped Anglo-American extradition treaty aimed at stripping political immunity from suspected terrorists who operate in Northern Ireland and seek refuge in this country.

The Irish National Caucus, a Washington-based group dedicated to influencing U.S. government policy toward Northern Ireland, has sent a direct-mail package to 250,000 Irish Americans attacking the treaty and urging them to write to key senators in a bid to sink the proposed accord.

On the outside of the envelope are the words, "The British are coming, the British are coming." It goes on: "The British are indeed coming, this time into the United States, and it is here that we must meet and beat them."

The new accord and the campaign against it provide further proof that the Irish-English battle is often fought on American soil.

Part of a get-tough policy toward international terrorism by President Reagan and Prime Minister Margaret Thatcher, the yet unratified treaty would revise a 13-year-old extradition treaty. A key change is the removal of the "political exception" clause, which would mean that people wanted for violent crimes in the United Kingdom could not successfully plead in a U.S. court that they had a political motive.

The supplemental treaty cannot take effect unless the Senate ratifies it by a two-thirds vote. Therefore, a vigorous lobbying campaign has been mounted by the Irish National Caucus, the Ancient Order of Hibernians and the Irish American Unity Conference.

Father Sean McManus, national director of the caucus, said that it opposes the proposed treaty because "it legitimizes English rule in Northern Ireland, it takes away authority from the U.S. courts and it returns people to Northern Ireland where they have no hope of a fair trial."

McManus said the direct-mail campaign had cost $65,000 and adopted computer techniques of private companies to pinpoint Irish-American households. These were added to lists built up since the caucus opened in Washington in 1978.

"Now we have a budget of $2 million a year . . . all from private American donors," said McManus, a member of the Redemptorist order who was forced to leave the United Kingdom in 1972 after pressure from the British authorities. McManus, who then spoke out in support of the Provisional Irish Republican Army, said the caucus is nonviolent and does not support violence by groups in Northern Ireland.

The Caucus was one of several Irish republican groups lobbying at a Senate Foreign Relations Committee hearing Wednesday, where legislators and academics vented their opposition to the treaty but concentrated their fire on its alleged antilibertarian content.

"In the context of the history of America as a nation of immigrants, conceived in revolt and as a people consequently sympathetic to struggles for self-determination, the right to refuse extradition of political offenders has become the hallmark of our democracy," said Rep. William J. Hughes (D-N.J.), chairman of the subcommittee on crime of the House Judiciary Committee, in written testimony.

"The 'political offense' exception finds its origins in the writings of Jefferson, Marshall, Webster and Mill and should not be lightly discarded," Hughes said.

Former senator Eugene McCarthy told the committee that he opposed the treaty not only because of the U.S. tradition of law and order but also on foreign policy grounds.

"You may be extraditing people into a system of justice where human rights are even less protected than they are now," McCarthy said.

In Northern Ireland, alleged terrorists are tried before a judge alone in what are known as the "Diplock courts," named after the author of a report recommending them.

McCarthy said there was "no real evidence of a rash of extradition requests" from the British government seeking suspected Irish terrorists or evidence that many terrorists were getting away.

The State Department's top legal adviser, Judge Abraham D. Sofaer, testifying at a Foreign Relations Committee hearing Aug. 1, said that in four cases since 1979, federal courts ruled that the Irish Republican Army activities involved constituted political offenses. These cases, he said, demonstrated a need for the revised treaty.

The British government also argues that the new supplemental treaty reflects practice in Europe where, under the European Convention of 1977, certain violent crimes such as hijacking do not come under the "political exception" clause.