WASHINGTON (Reuters) - Republican lawmakers in U.S. Congress on Wednesday rejected two Democratic amendments to provide nearly $1 billion in funding for bipartisan legislation intended to combat America's opioid epidemic, aides said.

The rejection, which came during a meeting of U.S. House of Representatives and Senate negotiators on the measure, could undermine Democratic support for final legislation that could come up for a vote in the House of Representatives as early as Friday. The House and Senate each passed its own versions of the legislation in the last few months.

House and Senate Democratic lawmakers are now withholding their support for the final version of the bill, according to two Democratic aides.

Democratic opposition could prove to be an obstacle, especially in the Senate, where lawmakers need 60 votes to move legislation forward.

Two amendments, from House and Senate Democrats, would have provided $920 million up front to finance program grants for states. Aides said the programs otherwise require completion of a lengthy appropriations process that could stall in a congressional session shortened by this year's election campaign.

Republican aides were not immediately available for comment. But House Republicans are separately considering a health spending measure that would provide more than $581 million to address opioid abuse.

House Democratic Caucus Chairman Xavier Becerra said on Wednesday it was only the latest instance in which legislation with bipartisan backing fell short on funding. "It's becoming a bit of a joke to say we're concerned and want to do something, when the only thing we'll do is shell of what it actually takes," he said.

(Reporting by David Morgan; Editing by Diane Craft)