U.S. fiscal talks stumble as lawmakers race against time

* Chaos in Congress as deadline nears on fiscal impasse

* Two plans by House Republicans collapse amid opposition

* Senate talks resume after waiting for House plan

* Fitch Ratings warns it could cut U.S. credit rating

By Richard Cowan and Thomas Ferraro

WASHINGTON, Oct 15 (Reuters) - Efforts by lawmakers to stop a U.S. debt default were in disarray on Tuesday with just over a day before the government's authority to borrow money lapses, risking the Treasury's ability to pay bills and creditors.

Chaotic negotiations to end the U.S. fiscal impasse failed to produce a deal, and left Congress and President Barack Obama desperately searching for a way to reopen the government and raise the country's debt limit ahead of a Thursday deadline.

Both measures require approval by the U.S. Senate and the House of Representatives. In the Democrat-led Senate, leaders of both parties resumed negotiations, but the Republican majority in the House appeared to be even more divided than usual.

In a span of a few hours, two plans floated by Republicans in the fractious House collapsed for a lack of support and Senate negotiations that were close to an agreement were suspended to await a solution from the House.

But on Tuesday evening, spokesmen for Senate Majority Leader Harry Reid and Senate Republican leader Mitch McConnell said their talks were back on. A spokesman for Reid said the two leaders "are optimistic that an agreement is within reach" but offered no timetable.

The uncertainty led Fitch Ratings to warn it could cut the sovereign credit rating of the United States from AAA, citing the political brinkmanship over raising the federal debt ceiling.

Fitch's action emphasized how close to an economically damaging default Washington has come, and sent U.S. stock futures lower.

House Republicans twice tried to come up with a new compromise but failed to satisfy Obama, Senate Democrats or Tea Party conservatives who are determined to win changes to the president's signature healthcare law before they will agree to concessions on the budget.

The first House Republican attempt was shot down in a closed-door meeting that had begun with members singing the hymn "Amazing Grace."

The second plan was scuttled hours before it was expected to hit the House floor for a vote after the influential Heritage Action for America, a conservative group, urged a "no" vote because it did not do enough to stop Obama's healthcare law.

Heritage said it would consider the plan a "key vote" in evaluating whether to back candidates in next year's congressional elections. Shortly after that warning, a committee hearing scheduled to set rules for the debate was postponed and the plan put in limbo.