* Republican-led House likely to be particularly resistant
* Enhanced background checks seen having best odds
* Outside groups gear up to put pressure on Congress
By John Whitesides and Thomas Ferraro
WASHINGTON, Jan 16 (Reuters) - President Barack Obama's proposals to curb gun violence face a difficult path through a sharply divided U.S. Congress, where the biggest gun-control fight in decades looms on an issue that has long been one of the most divisive in American politics.
Obama's plan sets up a showdown between a gun-control movement re-energized by the massacre of 20 children and six adults last month at a Connecticut school, and a powerful gun-rights lobby led by the National Rifle Association, which has blocked new action on gun control for almost two decades.
Members of both parties said Obama's call for expanded background checks for all gun buyers had the best chance of surviving the partisan fight in Congress. Just as clear is that Obama's pitch for Congress to reinstate the ban on military-style assault weapons that expired nearly a decade ago is unlikely to go anywhere.
The odds for a third Obama proposal, to limit ammunition clips to 10 rounds, seem to fall somewhere in between, lawmakers and analysts said on Wednesday.
"If you look at the combination of likelihood of passage and effectiveness of curbing gun crime, universal background checks is at the sweet spot," said Democratic Senator Charles Schumer of New York, a heavily Democratic state where newly enacted gun laws for background checks and ammunition limits mirror several of Obama's federal proposals.
For the Democratic president, the obstacles in Congress are large. In the Republican-led U.S. House of Representatives, Speaker John Boehner faces pressure from conservatives to resist even bringing a gun-control bill to the floor.
That is why the gun legislation is almost certain to rise from the Democratic-led Senate. Even there, overcoming procedural hurdles will require supporters of Obama's plan to win over several Republicans while gaining backing from a few Democrats who have supported gun rights and face re-election next year in conservative states.
Democratic Senators Tim Johnson in South Dakota, Max Baucus in Montana, Mary Landrieu in Louisiana, Mark Begich in Alaska and Mark Pryor in Arkansas are from conservative states with high levels of gun ownership, and all face re-election next year.
Begich, for one, has expressed reservations about a new ban on assault weapons.
The political calculus is such that Obama's plan "is not just dead in the House, it is on life support in the Senate before it even arrives," said Republican strategist Ron Bonjean, a former Capitol Hill aide.
But Democratic strategist Phil Singer, also a former Hill aide, said there was a chance that public pressure stemming from outrage over the Newtown, Connecticut, shootings could force Republican leaders to reconsider their opposition to at least some of the measures - or risk being painted as unreasonable and beholden to the gun lobby.
"It will be very difficult and it will require a tremendous amount of work, but I think that there is a decent chance that it could happen," Singer said. "At this point, it is irresponsible to rule out getting it done. The process has only just begun. I think there is a path forward."
LEAHY PROMISES ACTION
Some analysts were skeptical that anything could be accomplished in Congress, which faces a crush of other legislative priorities, including a looming battle over the budget and raising the U.S. borrowing limit.
Vermont Senator Patrick Leahy, Democratic chairman of the Senate Judiciary Committee and a gun owner, said he would begin hearings on Obama's proposals in two weeks and voiced support for closing loopholes that allow two of every five gun sales to occur without a background check.
"I would point out that I have a track record of getting legislation passed. There are some who say nothing will pass. I disagree with that," Leahy said after a speech on Wednesday at Georgetown University Law Center.
"I think we can tighten up our background checks. There are a number of things we can do," he said.
Senate Majority Leader Harry Reid, who has publicly expressed doubts about the prospects for an assault weapons ban, said he was committed to ensuring the Senate would consider a gun bill and that "all options should be on the table moving forward."
FOR OBAMA, A NEW CAMPAIGN
Gun-control supporters said the public climate had changed since the Newtown shooting, with polls showing rising support for many of Obama's proposals.
New groups led by New York Mayor Michael Bloomberg and former U.S. Representative Gabrielle Giffords, who was wounded in a mass shooting in Arizona in 2011, are countering the NRA's lobbying efforts.
The grass-roots campaign structure that propelled Obama to re-election in November will also be used to rally support for the gun-control plan among the president's supporters.
Former White House spokesman Robert Gibbs said the campaign's volunteers and email contacts could be put to work against the NRA's extensive lobbying efforts, and that Obama would go into campaign mode to sell his proposals.
"He's got to get out of the White House and travel the country," Gibbs said on MSNBC. "He's got to make his case directly to the people."
Gibbs said it was time to use the campaign's list of supporters for something more than an election. "If the NRA's got a list, then Obama for America has a bigger list," he said.
On Wednesday, Obama made an emotional appeal for support, using imagery of children at risk and assuring gun owners their constitutionally protected right to firearms would not be restricted by his proposals.
"This will not happen unless the American people demand it," he said.
"If parents and teachers, police officers and pastors, if hunters and sportsmen, if responsible gun owners, if Americans of every background stand up and say, 'Enough: We've suffered too much pain and care too much about our children to allow this to continue' - then change will come," Obama said. (Editing by David Lindsey and Peter Cooney)