WASHINGTON - A bipartisan proposal to expand background checks to more gun buyers seemed in jeopardy Monday as a growing number of Republican senators expressed opposition to the proposal, perhaps enough to derail it. But there was plenty of time for lobbying and deal-making to affect the outcome, and the sponsors seemed willing to consider carving out at least one exemption in an effort to drum up votes.

The White House said President Barack Obama was calling lawmakers, as both sides hunted support for a nail-biting showdown.

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 compromise last week with Sen. Patrick Toomey, R-Pa., on which the Senate was preparing to vote.

In one sign of the bargaining underway, Manchin and Toomey seemed willing to consider a change to their deal that would exempt gun buyers from background checks who live hundreds of miles from licensed firearms dealers, said one Senate aide.

The change might help win support from senators from Alaska and perhaps North Dakota, said the aide, who spoke on condition of anonymity to describe private discussions.

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, nine Republican senators from that group said they would oppose the Manchin-Toomey plan and one was 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, minds on both sides 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 spokesman Caley Gray 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 gun control debate last week: Sens. Mark Begich of Alaska and Mark Pryor of Arkansas both said they are still deciding.

Background checks, designed to keep guns from criminals and the seriously mentally ill, 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.

"There's no debate that that's not an infringement of the Second Amendment" right to bear arms, said Toomey as he and Manchin touted their measure on the Senate floor.

White House spokesman Jay Carney said the gun legislation was "an absolute priority" and said Obama has been contacting senators, though he declined to say which ones.

But Carney said the vote would be "a difficult challenge." He said that because the Senate had voted last week to begin debating the measure "does not mean we have gotten to where we need to be, which is passage of legislation that is commonsense and that will reduce gun violence in America."

The White House originally had hoped for much more, including a ban on military-style rifles and high-capacity ammunition magazines.

The National Rifle Association said it was running an ad on cable television's Sportsman Channel and online criticizing Mayors Against Illegal Guns for running an ad showing a man holding a gun unsafely as he describes his support for expanded background checks.

"Is it possible he's an actor?" the ad asks, just before showing the NRA's "Stand and Fight" slogan.

Some relatives of the victims of the Connecticut families are planning a return trip to Capitol Hill this week to meet with senators they weren't able to visit on their lobbying trip last week. That trip was partly credited with helping move the Senate to debate the gun bill.

Also scheduled to be lobbying lawmakers this week are former Rep. Gabrielle Giffords, D-Ariz., and her husband Mark Kelly, the retired astronaut. She was severely wounded in a 2011 mass shooting in Tucson, Ariz.

The Manchin-Toomey deal also would expand some firearms rights, easing some restrictions on transporting guns across state lines and protecting sellers from lawsuits if buyers pass a background check but later use a gun in a crime.

The compromise is an amendment to broader gun control legislation to strengthen laws against illegal gun trafficking and to slightly increase school security aid.

Associated Press writer Nedra Pickler contributed to this report.