Washington - Pressed by shooting victims and relatives of Americans slain in gun violence, the Senate on Thursday voted to begin an emotionally and politically charged debate on gun safety proposals as advocates of new laws overcame a Republican filibuster threat.

The strong majority in favor of considering legislation that would expand background checks and increase the penalties for illegal gun sales reflected the power of a lobbying campaign by parents of students killed in Newtown, and by others who persuaded reluctant lawmakers to back them in an initial fight that looked lost just last week. The vote was 68-31.

"It's remarkable," said Sen. Christopher S. Murphy, D-Conn., whose nascent Senate career has been devoted to gun safety. "You can't turn a corner in the Capitol this week without meeting a family of a gun violence victim. It's hard to say no to these families."

But the victory could be short-lived. The vote in no way guaranteed passage of the gun measure; some Republicans and Democrats who voted for this initial step made clear they are not committed to supporting any final measure, even if they agreed to allow the debate.

"I am not sure I could have the courage to do what they did," said Sen. Richard M. Burr, R-N.C., who met with family members of the Newtown victims Wednesday. "It really does have an impact." Burr voted to debate the bill, but said he was unlikely to go further. "Is there anything I've seen so far that would move me to vote for new gun laws?" he said. "No."

The coming weeks and even months will test both the resolve and the stamina of the families, who are both the best advocates for their cause and, in many ways, least equipped for its struggle.

"Every day is hard for me," said Mark Barden, the father of Daniel, who was killed at Sandy Hook Elementary School. "Making lunch for my kids is hard for me. Sleeping is hard. Waking up is hard. That being said, I just feel I need to be doing this."

The bill will again need 60 votes to end the debate after consideration of contentious amendments offered by both supporters and opponents of new laws. Opponents of the measure could also try to filibuster individual amendments,

Should the bill reach the stage where it could pass with a simple majority, it would still face a challenge. Though Democrats control 55 seats, many from conservative states who face re-election campaigns next year have indicated that they do not intend to vote for the bill, meaning Republican votes could be required to put it over the top.

Twenty-nine Republicans opposed bringing the measure to the floor, along with two Democrats. Sixteen Republicans joined 50 Democrats and two independents in voting to proceed to consideration of the legislation. The two Democrats who voted against measure are both up for re-election in tough states in 2014: Sens. Mark Begich of Alaska and Mark Pryor of Arkansas.

Next week, the Senate is expected to begin reviewing the bill in earnest by voting on an amendment offered by Sens. Joe Manchin III, D-W.Va. and Patrick J. Toomey, R-Pa., that would expand background checks to cover unlicensed dealers at gun shows as well as all online sales. It would also maintain record-keeping provisions that law enforcement officials find essential in tracking guns used in crimes.

This amendment would replace the background check provision of the original legislation, which would also create harsher penalties for the so-called straw purchasing of guns, in which people buy guns for others who are not able to legally. Subsequent amendments, dealing with mental health, a ban on assault weapons and other issues, are expected in the days ahead before a vote on the overall measure.

The omnipresence of the families this week, encouraged by President Barack Obama, and former Rep. Gabrielle Giffords, appeared to stretch even toward the House, where the journey will be even more difficult should the Senate pass legislation.

Speaker John A. Boehner, for instance, who earlier in the week reacted icily to the idea of new gun legislation, referred to the advocates Thursday after the Senate vote. "Listen, our hearts and prayers go out to the families of these victims," Boehner told reporters. "And I fully expect that the House will act in some way, shape or form. "

Alternating between moments of intense privacy - they wept openly in the offices of senators but would not say whom they met with - and their desire to promote their cause, family members inhabited a strange world of boom mikes and cameras and procedural votes. Senators laughed and visited on the floor, as they sat in the gallery hovering above them, listening to the clerk call each vote.

Shortly after the Senate vote, Obama spoke on the phone with Newtown family members to thank them for their advocacy efforts, saying "it wouldn't have been possible without them," said his spokesman, Jay Carney.

History has shown that family members of those killed in natural disasters, terrorist attacks and other tragedies have been able to affect public policy, most recently those of people killed on Sept. 11, as well as people suffering illness in the aftermath.

"I think that it will make a difference," said Kirsten Gillibrand, D-N.Y. "It worked when we had the 9/11 legislation on the floor of the Senate. We had the first responders, the survivors, come to Washington, talk to senators, talk to House members, tell their personal stories, tell about the horrible disease that they were fighting and that they had nowhere to turn."