* Resist political pressure to vote no, Manchin tells senators
* Debate over Senate plan has become a race to 60 votes
* Pennsylvania's Toomey: 'Can't we take a very modest step?'
Democrat Joe Manchin and Republican Pat Toomey defended their plan to expand background checks for gun buyers on Monday, as Manchin opened the U.S. Senate's debate on the proposal by urging colleagues to "make a difference" and resist political pressure to reject the measure.
Manchin's call for bipartisanship on one of the nation's most divisive issues came as the background checks plan that he and Toomey devised was headed to a close vote in the Senate later this week.
After an intense, emotional week of lobbying that included family members of victims from the Newtown, Connecticut massacre visiting Capitol Hill to call for action, both sides of the gun-control debate scrambled for votes on a crucial piece of President Barack Obama's gun-control legislation.
Manchin, who is from West Virginia and has long been a strong supporter of gun rights, sought to assure others that the background checks plan would not limit law-abiding citizens' access to guns. He also urged senators to look beyond politics in voting on the plan.
"'No' is the safest vote as a congress person or senator, I understand that. I can vote 'no' on about everything and be fine," Manchin said of the plan, which has drawn opposition from the National Rifle Association and other gun-rights proponents, but is supported by two other gun owners' groups.
"I want to make a difference. I want to do something," Manchin said, saying that members of Congress should not be so worried about "all of the outside pressure, and maybe getting elected, maybe getting campaign funds."
The proposal to expand criminal background checks to include sales made online and at gun shows represents Obama's best hope for meaningful gun-control legislation in the aftermath of the killings of 20 children and six adults at an elementary school in Newtown, Connecticut, on Dec. 14.
The deal between Manchin and Toomey, designed to keep guns out of the hands of criminals and the mentally ill, would be the centerpiece of gun-control legislation that also would tighten laws on gun trafficking and increase funding for school security.
Like Manchin, Toomey has been a staunch defender of Americans' constitutional right to bear arms. The Pennsylvania senator said he did not consider the background checks provision to be gun control, while acknowledging that there was no way to prevent all criminals or mentally unstable people from securing firearms.
"But can't we take a very modest step to make it more difficult if we can do it in a way that does not infringe on the Second Amendment rights of law-abiding citizens?" Toomey asked.
THE NUMBERS GAME
The Manchin-Toomey amendment needs 60 votes to overcome procedural hurdles erected by Republican opponents in the Senate, where Democrats control 55 seats.
Some Democrats up for re-election in conservative states next year also are expected to oppose the measure. Democrats Mark Pryor of Arkansas and Mark Begich of Alaska, both facing tough re-election battles, voted with conservative Republicans last week in an unsuccessful effort to block debate on the measure.
That means the Manchin-Toomey plan will probably need the backing of at least a half-dozen Republicans to clear the chamber. The Senate voted 68-31 last week to take up the broader legislation, with 16 Republicans joining 50 Democrats and two independents in supporting a debate on the bill.
But seven of those Republicans have said they will not support the background checks plan. Aside from Toomey, the only Republicans who support the deal so far are Mark Kirk of Illinois and Susan Collins of Maine. John McCain of Arizona said on Sunday he was "favorably disposed" to vote for it.
'TOUGH TO SAY'
"It's tough to say right now if we've got enough votes" to pass the Manchin-Toomey amendment, a Senate Democratic aide said.
More controversial aspects of Obama's gun-control plan - including a ban on rapid-firing "assault" weapons like the one used in Connecticut and limits on the capacity of ammunition magazines - appear to have a slim chance in the Senate.
Even if the background checks bill clears the Senate, it would face tough opposition in the Republican-controlled House of Representatives.
Obama, Vice President Joe Biden and other administration officials have pushed for the bill in a series of public events; Obama brought the Newtown family members to Washington aboard Air Force One after he visited Connecticut.
The Newtown families had several emotional visits with lawmakers. During their meeting with Manchin, the senator was moved to tears.
The NRA gun lobby group opposes the measure and has warned members of Congress it will include their votes on the Manchin-Toomey amendment in the ratings it compiles on each lawmaker and sends to members.
Manchin and Toomey have been contacting their colleagues in a hunt for support. Former U.S. Representative Gabrielle Giffords, who was badly wounded in a 2011 mass shooting in Arizona, will visit Washington this week to push for gun-control legislation.
The International Association of Chiefs of Police endorsed the background checks deal on Monday.
"Background checks work. Keeping firearms out of the hands of prohibited purchasers is a key factor in reducing gun violence in our communities and protecting our officers," said Craig Steckler, president of the police chiefs' group.
The background checks provision picked up an unexpected boost over the weekend with the backing of the Citizens Committee for the Right to Keep and Bear Arms, which bills itself as the second-largest gun-rights group behind the NRA with 650,000 members and supporters.
The group cited some of the sweeteners in the deal for gun-rights supporters, including a provision that would make licensed interstate sales easier and ban the creation of a gun registry, one of the frequent fears cited by groups such as the NRA.
Those provisions, along with the possibility that some Republican amendments to strengthen gun rights could clear the Senate, add to the unpredictability of the outcome of the Senate debate.
"It isn't perfect, but it's certainly a long, big, heavy step forward. Expanding background checks to cover gun shows and Internet sales is common sense," Senate Democratic Leader Harry Reid said.