WASHINGTON - It was hard enough getting one Democrat and one Republican to work out a compromise on expanding background checks of gun buyers.
Now comes the vote on whether the amendment by Sens. Joe Manchin and Pat Toomey can win Senate approval -- the next step on the tortuous path of gun legislation in the aftermath of the Newtown school massacre.
The Senate is expected to take up the gun package on Tuesday, starting with the amendment drafted by Manchin, D-West Virginia, and Toomey, R-Pennsylvania.
Both are self-professed gun lovers who have gotten high marks from the powerful National Rifle Association for their voting records on firearms issues.
However, the NRA opposes their compromise, casting doubt on whether it can get the 60 Senate votes necessary to pass under the filibuster threshold imposed by Republicans.
Neither senator predicted victory when discussing the measure on Sunday on CNN, with Toomey saying it was an "open question" on whether it would pass.
The overall gun package received 68 votes last week -- including 16 from Republicans -- to bring it to the Senate floor.
Far fewer Republicans are committed to backing the Manchin-Toomey amendment or supporting the final gun package once the debate is over.
So far, four GOP senators have declared they will support the Manchin-Toomey compromise or were leaning toward backing it.
With at least two moderate Democrats from pro-gun states known to oppose the measure, it needs at least seven GOP votes to have any chance of passing.
The amendment will be one of perhaps dozens to be debated on the broader package of gun laws pushed by President Barack Obama following the December shooting of 20 first-graders and six educators at the Sandy Hook Elementary School in Newtown, Connecticut.
In debate expected to last two weeks, senators will consider efforts by both sides to either expand or weaken the gun package.
A push by Democrats for an updated ban on semiautomatic firearms modeled after military assault weapons, as well as limiting ammunition magazines to 10 rounds, is expected to fail.
Republicans will push an NRA proposal to make state concealed weapons permits acceptable throughout the country, a concept opposed by many Democrats.
The package also contains measures to crack down on gun trafficking and straw purchases, and finding ways to improve school safety.
Since the Newtown attack by a lone gunman, Connecticut and a few other states have passed tougher gun laws.
Polls show Americans favoring expanded background checks and other proposals intended to crack down on gun trafficking.
But more pro-gun states have passed laws easing firearms restrictions and the NRA has promised political retribution for legislators who back measures it opposes.
"This debate in some ways is underscoring just the extent to which there is a polarization in our society," Toomey told CNN, lamenting "the acrimony that has gotten into politics."
Asked if he was concerned about political repercussions, the conservative Republican said: "I'll just let the political chips fall the way they fall."
In a separate interview on CBS, Manchin acknowledged the power of the NRA, saying: "They won`t be with us on this and I just would hope that they would allow their members to see the facts and let them vote their conscience."
Both senators said part of the problem involves misconceptions about their compromise, and they urged colleagues to read the 49-page measure instead of relying on what they called erroneous political rhetoric by opponents.
Their amendment expands background checks to gun shows and all Internet sales, but exempts private transactions such as hunting rifles traded among family and friends.
It also makes it easier for hunters and sport shooters to transport their firearms across state lines. It would also require states and the federal government to provide records on criminals and the "violently mentally ill" to the national background check system, addressing a criticism by the NRA and other opponents of gun laws that the existing system lacks substantive information.
In addition, the plan calls for a new National Commission on Mass Violence to report in six months on "all aspects of the problem, including guns, school safety, mental health, and violent media or video games."
The NRA opposes the compromise as a possible first step toward a national gun registry and therefore a violation of the constitutional right to bear arms.
However, smaller pro-gun groups have come out in favor of the Manchin-Toomey amendment, indicating a rift in the gun lobby over the emotional issue of taking legislative action in response to the Newtown killings.
On Sunday, Manchin and Toomey noted their proposal includes a specific prohibition against a national gun registry as well as criminal penalties for misusing background check information for that purpose.
"There are going to be some people who said you didn`t go far enough, some are saying you went too far," Manchin told CBS. "But if they`ll look at the bill, what we did, we did right."
In a statement on Sunday, moderate Republican Sen. Susan Collins of Maine said she would support the compromise because it exempted private family gun transactions from needing a background check, calling it a "common-sense approach" that required the screening only for commercial transactions.
Even if the compromise passes and the Senate proceeds to approve a broader gun package, it was unclear if the Republican-led House would go along.
House Speaker John Boehner, R-Ohio, has stopped short of promising a vote, saying only that the chamber would look at whatever the Senate sends over.
Obama has made the gun legislation a major focus of his second term, holding events across the country to push for congressional action and public pressure.
Last week, the mother of one of the Newtown schoolchildren killed in December delivered the weekly presidential address instead of Obama.
Francine Wheeler, whose 6-year-old son Ben died in the attack by a lone gunman, noted that since the December 14 killings, "thousands of other Americans have died at the end of a gun."
"Thousands of other families across the United States are also drowning in our grief," Wheeler said. "Please help us do something before our tragedy becomes your tragedy."
The issue has raised attention on overall gun deaths in the country, with supporters of tougher gun laws noting that more than half all homicides involve firearms and U.S. levels are higher than other countries.
On Saturday night, a man shot himself to death during a NASCAR event sponsored by the NRA in Texas.