Despite a recent surge in momentum for gun control measures, final passage of new restrictions remains very much in doubt as opponents in the House are gearing up to either block the provisions recently adopted by the Senate or dramatically reduce their effect.

House Republican leaders last month reversed years of opposition to gun restrictions and embraced mandatory background checks at gun shows, child safety locks and other measures approved by the Senate. In the wake of the Littleton, Colo., shootings, House leaders were anxious to stay ahead of the Democrats and keep their chamber from dissolving into the chaos that gripped the Senate during its debate last month.

But with grass-roots activists now mobilized on both sides of the issue and many lawmakers still resistant to even modest gun control measures, the House remains deeply divided on how to proceed as it reconvenes today, according to lawmakers and congressional aides.

Some conservative Republicans, such as Judiciary Committee member Robert L. Barr Jr. (Ga.), are already planning to try to weaken the gun safety provisions slated for a committee vote this week.

"I continue to believe Congress should address the problem of youth violence with a carefully considered, comprehensive strategy, rather than a knee-jerk reaction to tragic incidents," Barr said in a statement. "If, as seems to be the case, the decision is made to rush legislation forward, I have every intention of working to improve it via the amendment process."

The House traditionally has been more resistant to gun control than the Senate, and House GOP leaders are struggling to craft a compromise that can placate many traditional gun control opponents while attracting enough supporters to ensure passage. According to lawmakers and aides, between 175 and 180 members solidly oppose gun restrictions, including 30 moderate-to-conservative Democrats from states such as Michigan and Wisconsin.

Congressional aides warn that if the final package developed by the House includes too much gun control, a majority of Republicans and a substantial number of Democrats will vote no. If there's too little gun control, a substantial number of liberal Democrats could join gun control opponents to defeat the bill.

"What you need is a Goldilocks bill," said one House leadership aide. "It's going to have to be just right."

GOP and Democratic aides to the Judiciary Committee have been negotiating over the kind of gun safety provisions the panel should vote on this week but so far have failed to reach an agreement. Republicans are backing a mandatory 24-hour background check for gun show sales, for example, while Democrats support the 72-hour waiting period adopted by the Senate.

House Speaker J. Dennis Hastert (R-Ill.) also has promised to support legislation requiring the sale of child-safety locks and prohibiting individuals under the age of 21 from purchasing guns.

According to Handgun Control Inc. President Bob Walker, "virtually everyone is accepting background checks at gun shows. Of course, the devil is in the details."

Rep. Rosa L. DeLauro (D-Conn.) warned that Democrats would be quick to attack any attempts at "watering down" the Senate-passed restrictions. "I think people are tired of loopholes written by the NRA," she said, referring to the National Rifle Association.

DeLauro and Rep. Nita M. Lowey (D-N.Y.) said they plan to target the roughly 40 like-minded Republicans whom Lowey described as "critical" to passage of new gun controls.

But Hastert spokesman John Feehery described measures like the 24-hour gun show waiting period as "reasonable," adding that the House can adopt gun safety measures as long as Democrats are willing to compromise.

"It's going to be easy to get common-sense legislation," he said. "It's going to be hard for liberal Democrats who overreach."

Many Republicans are concerned about alienating their most loyal voters, many of whom were angered by the Senate's decision to reverse course and adopt gun restrictions.

Rep. Chris Cannon (R-Utah), another Judiciary Committee member, received an angry call from his mother over what she viewed as "the capitulation of the Senate." Cannon, who supports a 24-hour background check for gun show sales, said he and other Republicans face a challenge in educating voters because "the mess on the Senate floor was deeply disconcerting, and word went out among conservatives they were going to lose their gun rights."

President Clinton and other Democrats had pushed for an earlier vote on gun measures, arguing that otherwise the NRA would have more time to regroup, and there were some signs that their warnings were accurate. Rep. Bart Stupak (D-Mich.), who belongs to the NRA but said he could support a 72-hour background check and trigger locks on guns, received two separate NRA mailings this week specifically targeting him. His working-class district includes the Upper Peninsula and the other northern parts of Michigan.

One of the NRA mailings, which directed local residents to contact Stupak's office, compared the Senate gun bill to Hillary Rodham Clinton's failed health care plan and described the upcoming House vote as "the most critical gun vote in over five years."

"I'm going to have a long talk with myself in the mirror tonight," Stupak joked, adding quickly that the lobbying could make it more difficult for House leaders to pass meaningful legislation. "I think they're going to have trouble. I've looked at the NRA mailings, and they're pretty persuasive."

Cannon, who has since placated his mother, is planning to offer an amendment ensuring the government keeps no record of gun owners after they clear the needed background checks. Despite the furor, he said he believes some form of gun safety legislation will pass the House.

"I have yet to find a constituent who doesn't want some reasonable steps taken to keep guns out of the hands of people who shouldn't have them," he said.