The Republican Senate majority is in jeopardy of being washed away by a torrent of resources amassed by Democratic challengers in the third quarter, with GOP incumbents bracing to be vastly outspent in the final month of the campaign.

Democratic candidates are raising hundreds of millions of dollars. Even previously overlooked challengers saw their coffers swell with contributions from energized grassroots liberals in July, August, and September, turning sleepy Senate races into a potential nightmare for sitting Republicans and the party’s precarious three-seat majority.