The White House said President Barack Obama was calling lawmakers, looking for support.
As of Monday evening, some senators were saying the vote now appeared likely late this week, rather than midweek as top Democrats have hoped. Such a delay would give both sides more time to find support.
"The game hasn't even started yet, let alone over," said Sen. Joe Manchin, D-W.Va., who reached a background check deal last week with Sen. Patrick Toomey, R-Pa., on which the Senate was preparing to vote.
At stake is what has become the heart of this year's gun control drive in response to December's killing of children and staff at an elementary school in Newtown, Conn. Supporters consider a broadening of the buyers subjected to background checks to be the most effective step lawmakers can take, and Obama urged near universal checks in the plan he unveiled in January.
Sixteen Republicans voted last week to reject an effort by conservatives that would have blocked the Senate from even considering a broad bill restricting firearms. With that debate underway, Democrats hope to win enough supporters from this group to gain passage of the first amendment to that bill -- the compromise between Manchin and Toomey, which expands background checks but less broadly than Obama has wanted.
By Monday evening, eight Republican senators from that group said they would oppose the Manchin-Toomey plan and two were leaning against it. Combined with the 31 senators who voted against debating the overall gun bill last week, that would bring potential opponents of expanding background checks to 41 -- just enough votes to block the Senate from considering the compromise.
"I'm not going to vote for it. It's not the right thing to do," said Sen. Saxby Chambliss, R-Ga., who was among the 16 who voted last week to allow the debate to begin.
But in the heated political climate and heavy lobbying certain in the run-up to the vote, it was possible that minds could change.
Opponents say expanded checks would violate the Constitution's right to bear arms and would be ignored by criminals. They are forcing supporters of the background check plan to win 60 of the Senate's 100 votes, a high hurdle.
Fifty Democrats and two Democratic-leaning senators voted last week to begin debate. If all of them support the Manchin-Toomey plan -- which is not guaranteed -- they would still need eight additional votes.
So far, three Republicans who backed beginning debate have said they will vote for the Manchin-Toomey plan: Toomey and Sens. Mark Kirk of Illinois and Susan Collins of Maine. A fourth, John McCain of Arizona, said he is strongly inclined to do so.
Sen. Frank Lautenberg, D-N.J., missed last week's vote after saying he was suffering from muscle weakness, but a spokesman said he hopes to be in the Senate for votes this week.
Two Democrats, both facing re-election next year in GOP-leaning states, voted against beginning the debate last week: Sens. Mark Begich of Alaska and Mark Pryor of Arkansas both said they are still deciding.
Background checks, are currently required only for sales handled by the nation's roughly 55,000 licensed gun dealers.
The Manchin-Toomey measure would extend that to sales at commercial venues like gun shows and online, while exempting other transactions like those between relatives and friends.
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