As much as it was a strike against first-time violent felons, the Assembly's passage today of "Jenna’s Law" was a political blow to the Assembly Speaker, Sheldon Silver -- an admission that the Democratic leader and his allies had badly miscalculated how potent an election-year issue it would turn out to be.

Under mounting pressure, the Democrats, who control the chamber, were dragged back to the Capitol to pass the bill sharply curtailing parole, legislation that many of them had hoped would never come to a vote.

One after another, Democrats rose on the floor -- sounding stung and angry and not quite believing their circumstances -- to complain that the Republioims had, of all things, used the issue as a campaign cudgel against them.

"Why are we here today?" asked Assemblyman Keith L. Wright, a Manhattan Democrat. "Because of an onslaught of cynical and distorted communications by the Republican Party about our record."

The surprise is that the Democrats could have been surprised.

Gov. George E. Pataki, who is running for re-election, began the year by declaring the measure to be his top priority for the year, and at every turn he 0 . W the Democrats up for failing to pass it. He foun d unexpectedly effective allies in Bruce and Janice Grieshaber, the parents of Jenna Grieshaber, who was killed in Albany last November and for whom the Republican Governor named his bill. The leader of the Republican minority in the Assembly, John J. Faso, made it clear that this would be a campaign issue.

For the Governor, this was a no-lose situation. He would either emerge from the year with a popular law that he had championed or with an easily understood campaign attack against the Democrats.

Today's vote gave him the result he preferred, adding to a law-and-order resume that is crucial to his aspirations for national office. In one term, Mr. Pataki had returned the death penalty to New York, lengthened prison sentences for many crimes and curtailed parole for repeat felons.

But until recently, Mr. Silver and many other Assembly Democrats were convinced that the parole issue posed no threat to them. In the last weeks of the regular legislative session, the Assembly passed a bill that included the restrictions on parole for violent felons but also increased education programs in prisons and would have allowed many nonviolent offenders to go through drug treatment rather than to prison.

Polls showed their alternative to be popular, and the Grieshabers endorsed it. Even though Mr. Pataki and the Republican majority in the Senate rejected the Assembly plan, Mr. Silver, of Manhattan, and his allies calculated that they had all the political cover they needed.

At one point, one of the Speaker's top aides even tried to persuade a reporter that the untold story of the year was how the Democrats had managed to wrest the parole issue away from the Republicans and turn it in their favor.

But when the session ended on June 19 with no law enacted in the Assembly, the Grieshabers began complaining publicly that they had been had by Mr. Silver. They appeared with Republican challengers to Democratic Assembly members.

Mr. Pataki and his choice for lieutenant governor, Mary 0. Donohue, did the same, with the simple, pointed message that the Democrats had refused to abolish parole for violent felons. People turned up at political rallies wearing shirts with wanted posters of Mr. Silver.

Today, Assemblyman Roger L. Green, a Brooklyn Democrat, protested "the political climate and the tactics that have been used."

Assemblyman Joseph R. Lentol, a Brooklyn Democrat, said, "It is my hope that we will end the tradition of politicizing criminal justice issues."

He might as well have hoped for an end to partisan politics.

Today's vote was evidence not only of the Democrats' miscalculation but also of their fear that come November, they will founder in a Republican tide.

The Governor's popularity is high, no clear Democratic front-runner has emerged from among his would-be challengers and the Republican Party has taken aim at many incumbent legislators.

Democratic strategists fear the party could I up to 10 of its 97 seats in the 150-member Assembly. Many Democrats from those districts voted the bill today despite their private objections to it.

"If this was a normal year, a strong party, a secure conference,we could stand firm and not vote this bill," one Assembly member said on the condition of anonymity. "But we're running scared."

Yet none of these concerns are new. Many Assembly Democrats considered the Governor's race lost a year ago. Last winter, Mr. Silver made it clear that he intended to avoid confrontation with the Governor at all costs, for fear of a political backlash. "-

Still, today, many Democrats seemed stunned by the turn of events. Even'as they prepared to capitulate to the Governor, they could not resist the urge to keep arguing that their plan, including drug treatment and education, was superior.

Mr. Faso, the Republican Assembly leader, said, "The majority in this house only felt the heat; they never saw the light."