The Senate, ending a long and divisive struggle, yesterday ratified an extradition treaty with Britain that will make it easier for British authorities to gain custody of members of the outlawed Irish Republican Army who take refuge in the United States.

The treaty, signed in Washington on June 25, 1985, alters the longstanding "political-exception" doctrine in U.S. extradition policy under which federal judges could deny extradition of persons on the grounds that the crimes they were accused of were politically motivated.

The new treaty includes a list of violent crimes -- including murder, kidnaping, aircraft hijacking and the planting of explosives -- that are not to be considered political in extradition cases initiated by Britain.

The Senate ratified the treaty 87 to 10, then quickly approved by voice vote a two-year, $20 million package of economic aid for Northern Ireland.

The British sought the changes in the extradition treaty as a new weapon to combat attacks on British targets by the IRA in Northern Ireland, the scene of centuries of religious strife between Protestants and Roman Catholics. But opponents of ratification argued that the price of helping the British fight IRA terrorism was too high and would involve turning away from the historic U.S. role as a haven for political dissidents.

Sen. Christopher J. Dodd (D-Conn.), who voted in the Foreign Relations Committee to send the treaty to the Senate floor, reversed himself yesterday. He said failure to amend the treaty on the floor left the political-exception doctrine "little more than a footnote" in the accord.

"The scales of justice are tipped in favor of police power," Dodd said. "This treaty equates all political violence with terrorism. It serves notice that we are prepared to undermine the kind of political principles that have made our democracy the envy of the world."

In response, Sen. Thomas F. Eagleton (D-Mo.) said the violent crimes excluded from the political category "are not crimes of politics" and that their perpetrators are not "expatriate politicians."

"They are terrorists," Eagleton said. "Terrorism is the same whether you have swarthy cheeks and talk in Arabic or if you have pink cheeks and talk with a brogue."

The British agreed to several changes the Foreign Relations Committee made in the treaty, including allowing U.S. courts to guard against political or religious persecution in extradition cases and to consider the fairness of the British court system in deciding extradition cases.

But in the floor debate, which began Wednesday afternoon, the Senate turned back two attempts to weaken the treaty. By 87 to 9, the Senate rejected an amendment by Sen. Jesse Helms (R-N.C.) that would have retained as a defense against extradition a claim that alleged crimes were part of an armed rebellion against the military authority of the state.

Helms said he opposed the treaty because he feared it could set a precedent for countries such as Nicaragua to seek extradition of antigovernment rebels who take refuge in the United States.

The Senate also rejected, 65 to 33, an attempt by Sen. Alfonse M. D'Amato (R-N.Y.) to prohibit retroactive use of the treaty where U.S. courts had denied extradition of persons sought by the British.