Senate Republicans, joined by many Democrats, accelerated their criticism yesterday of President Clinton's decision to free a dozen Puerto Rican nationalists from prison, calling on Attorney General Janet Reno to testify on the matter and passing a resolution deploring the decision.

Meanwhile, the Clinton administration canceled an FBI official's planned appearance before a Senate committee and refused to provide Congress with documents on the subject. Privately, some White House officials said the affair has become a political fiasco, and they believe talking about it will make it worse.

"We're not going to win this one," said one prominent administration official.

The debate's tone has become almost purely political, as all sides agree there is no chance of reversing Clinton's decision to grant early prison releases to members of the FALN terrorist group. Eleven FALN members were freed Friday, and most have returned to Puerto Rico. Two others declined Clinton's offer, which required them to renounce violence. Another will be eligible for release in a few years, and two others already were out of prison.

The Senate voted 95 to 2 yesterday for a resolution calling the clemency decision "deplorable." The House passed a similar measure last week.

Even some usually loyal Democrats have grown impatient with Clinton's refusal to give a more thorough explanation in the face of opposition from the FBI and other law enforcement groups.

Sen. Charles E. Schumer (D-N.Y.), who voted for the resolution, said: "I have repeatedly requested information on these cases. I have been given no such information."

The president has said politics played no role in his decision, and he was swayed in large part by the lengthy terms--nearly 20 years in several cases--already served by the FALN members. But Republicans yesterday continued to fan suggestions that Clinton was trying to build support among Puerto Rican voters for his wife's Senate bid in New York.

That point was raised repeatedly at a sometimes emotional hearing before a Senate Foreign Relations subcommittee chaired by Sen. Paul Coverdell (R-Ga.), a sharp critic of the decision. Among those testifying were two former New York City police officers who were badly wounded and disfigured by FALN bombs in 1982. None of those granted clemency by Clinton was directly linked to bombings that caused injuries or fatalities, a point the president cited in explaining his decision.

Coverdell and other critics, however, noted that some of the 11 released on Friday had been caught building bombs. Other FALN members were linked to 130 bombings that killed six and maimed dozens, and critics such as Coverdell said all FALN members should be considered terrorists.

Originally scheduled to testify yesterday was Neil Gallagher, assistant director of the FBI's National Security Division. But the administration "pulled the plug" on Gallagher's appearance Monday night, Coverdell told reporters, adding: "I think it's pretty clear that the White House is behind this. You've got to wonder what would cause the White House to bypass a chance to explain themselves."

Assistant Attorney General Jon P. Jennings told Coverdell in a letter that the administration decided not to send a representative to explain the decision because granting clemency is "an exclusive presidential prerogative."

Coverdell and Foreign Relations Committee Chairman Jesse Helms (R-N.C.) sent Reno a letter saying, "It is completely unacceptable for the administration to refuse to discuss the president's decision to offer clemency to 16 convicted terrorists." The letter asked Reno to appear before the committee within two weeks.

Privately, several White House officials said the clemency decision caught Clinton's top political aides by surprise, giving them no chance to flag the potential political pitfalls. As criticism of the Aug. 11 announcement grew, Hillary Rodham Clinton said on Sept. 4 that the clemency offer should be withdrawn because the FALN members, at that point, had not agreed to renounce violence.

Clemency advocates got the attention of Charles F.C. Ruff, who recently left his post as White House counsel. Ruff assembled a detailed file on the 16 FALN members earlier this year and presented it to Clinton with a recommendation for clemency, officials said.