The Republican Party's two-year run in the majority of the U.S. Senate is at serious risk and may well end on Nov. 8, senior congressional aides said on Wednesday, blaming Donald Trump as a drag on down-ballot Republican candidates.

With 13 days to go before elections, several Senate aides from both parties privately warned of trouble for Republicans.

"Things are not good ... the Senate is gone," said one Republican aide who asked not to be identified in order to candidly discuss the turbulent outlook for the 2016 campaign.