WASHINGTON -- The Senate cleared a major procedural hurdle on Thursday as it voted to overcome a Republican filibuster effort and begin debate on a gun control package.

Senators voted 68 to 31 take up the bill put forward by Senate Majority Leader Harry Reid (D-Nev.) that would expand background checks to more gun buyers, form a national commission on mass violence, create a federal gun trafficking statute and enhance school safety measures. The procedural motion required 60 votes to pass, and Republicans had been dodging questions for days on how they would vote.

If the senators who voted "no" had prevailed, the Senate wouldn't have been able to debate the package, let alone amend it and vote on it.

There were some notable party splits. Two Democrats, Sens. Mark Pryor (Ark.) and Mark Begich (Alaska), joined the Republican filibuster effort. Sixteen Republicans sided with Democrats to vote to begin debate on the bill: Sens. Lamar Alexander (Tenn.) Kelly Ayotte (N.H.), Richard Burr (N.C.), Saxby Chambliss (Ga.), Tom Coburn (Okla.), Susan Collins (Maine), Bob Corker (Tenn.), Jeff Flake (Ariz.), Lindsey Graham (S.C.), Dean Heller (Nev.), John Hoeven (N.D.), Johnny Isakson (Ga.), Mark Kirk (Ill.), John McCain (Ariz.), Pat Toomey (Pa.) and Roger Wicker (Miss.).

The vote comes during a particularly intense week in the debate over how to stem gun violence. Families of Newtown, Conn., shooting victims have been on Capitol Hill meeting with senators to urge them to support the bill. During Thursday's vote, they sat in the balcony watching lawmakers cast their votes. Shortly after the vote, President Barack Obama called the families to congratulate them and said the Senate debate on gun violence "wouldn't be possible" without their efforts, White House press secretary Jay Carney said during his daily briefing.

The outcome was clearly a win for gun control advocates, but the bill still faces major challenges. For starters, its opponents are expected to use the full 30 hours allotted before debate can actually begin on the bill, which means senators can't begin offering amendments until next week. Once the amendment process does begin, Republicans are expected to put forward several measures designed to weaken the bill, including an amendment by Sen. Lindsey Graham (R-S.C.) that would gut it and replace it with a watered-down alternative. Beyond that, 60 votes will again be needed in order to end debate on the bill and move to final passage -- a key vote that the National Rifle Association announced it will score.

A Senate Democratic leadership aide said to expect about 10 amendments to the bill. The aide noted in an email that some Republicans are even threatening to object to take up the amendments all at once, which means "it could take 3-4 days to set up EACH amendment vote."

Still, the fact that the bill has now cleared the first procedural motion means it will receive a debate. Up until minutes before the vote, Republicans were urging their colleagues to vote against letting that happen.

"As of this very moment, not a single senator has been provided the legislative language" of the background checks piece of the bill, said Sen. Mike Lee (R-Utah), referring to a bipartisan deal cut on Wednesday on that provision. "That is the centerpiece of this measure. It is critical that we all know what's in the bill before we vote on it ... That's why I ask my colleagues to vote against cloture."

The gun bill "remains controversial" and "has little chance, if any, of going anywhere," argued Sen. John Cornyn (R-Texas). "I'm not going to vote to proceed to a bill that has not yet been written, no matter how well-intentioned it may be. We need to make sure that what we do is address the cause of this violence and to not come up with symbolic gestures."

Sen. Patrick Leahy (D-Vt.) chided lawmakers for trying to block even a debate on the gun bill.

"We are the 100 senators elected to represent more than 314 million Americans. We are accountable to them, not to special interests lobbies on the left or the right. They should not dictate what we do," he said. "I urge all senators to be less concerned with special interest score cards and more focused on fulfilling our oath to faithfully discharge the duties of our offices as United States senators."

[Read the text of the Senate gun control bill here.]

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