By Richard Cowan and David Lawder
WASHINGTON (Reuters) - President Barack Obama and his Republican opponents in Congress enter a crucial week in the "fiscal cliff" impasse with more than just differences over taxes to bridge: Also in the way is pervasive mistrust among members of Congress that discourages big concessions for fear the other side won't reciprocate.
That distrust is fueling doubts among Republicans and Democrats about relying on the other side to live up to any bargains struck now on deficit reduction in the future, Capitol Hill aides say.
Because resolution of the immediate cliff issues depends in part on commitments by both sides to a framework for overhauling the tax code and entitlement programs over the next year, an atmosphere of disbelief could impede any agreement on the cliff.
Nearly everyone - from Obama to most of the 535 members of the House of Representatives and Senate - say they want to avoid the cliff's $600 billion in harmful tax hikes and spending cuts set to kick in automatically in the new year, and replace them with more reasoned savings.
Obama wants the tax cuts that originated during the administration of President George W. Bush extended for middle class taxpayers only, while Republicans want them extended for all, including the wealthiest.
But widespread credibility issues could cause problems for Democrat Obama and Republican House Speaker John Boehner if and when they have to sell any agreement they forge to their parties in Congress.
The task for Boehner, who met Sunday with Obama, may be greater because many Republicans distrust each other as well as the Democrats.
The distrust between the parties is nothing new, having been built up over years of partisan recrimination. But it's now become a regular feature of debate on the cliff, on and off the floors of the House and Senate.
Some Democrats have spoken darkly of a hidden Republican agenda to gut, not just cut, social safety net programs.
Republicans, said Oregon's Peter DeFazio in a December 5 House floor speech, are acting under the "guise of deficit reduction," to "somehow kill Social Security, which they've never supported."
GOING OVER THE CLIFF?
Republicans, for their part, say they don't believe Democrats will follow through on promises to make spending cuts.
"The reason we haven't heard Democrat ideas for entitlement reform may be because they have no plans to cut or to reform entitlement spending at all," Republican Representative John Fleming of Louisiana declared on the House floor last week.
"This is just another game from their play book - raise taxes and increase spending, as always," said Fleming.
And some see behind the firm Democratic stance a desire to go over the cliff in order to discredit Republicans, who several recent polls have indicated will bear the lion's share of the blame.
"I think their whole game plan is take the economy off the fiscal cliff and then blame Republicans," Wisconsin Republican Sen. Ron Johnson said on CNN December 2.
And neither side trusts the other's math. While Obama maintains his plan will shrink deficits by $4.4 trillion over four years, the Republican staff on the Senate Budget Committee says it's more like $400 billion.
Doubts about the worth of promises are fueling a renewed argument over the federal debt ceiling which could greatly complicate resolution of the impasse over the cliff.
The nation's borrowing limit - which controls the government's capacity to borrow money to pay past debts - will need to be increased by the end of February. Under current law, an act of Congress is required to do that.
Obama is demanding, as part of the fiscal cliff negotiations, enhanced presidential authority over the debt limit, in order to stop Republicans from using it as leverage against the White House.
Senate Minority Leader Mitch McConnell of Kentucky and other Republicans are resisting, saying that without debt ceiling leverage, the White House won't make the cuts it's promising.
"Look," McConnell said in the Senate last week, "the only way we ever cut spending around here is by using the debate over the debt limit to do it."
More broadly, members well aware that Congress gets mired in gridlock on much lesser pieces of legislation, find it hard to conceive of Congress succeeding at the far more complex tasks of overhauling the tax code and entitlement programs.
The truth is that many members of Congress share the same low regard for Congress that the public expresses in surveys.
Indeed, a regular theme in floor speeches by members of both the House and Senate is dysfunction - their own.
(Editing, additional reporting and writing by Fred Barbash; editing by Todd Eastham)