The Senate Conservatives Fund, once affiliated with former Sen. Jim DeMint of South Carolina, is accusing McConnell of “quietly allowing” the Gang of Eight bill to move through the upper chamber. In an email to supporters, the fund’s executive director Matt Hoskins urged the Kentucky Republican not only to oppose the bill, but also to rally his entire caucus in opposition.

“A simple ‘No’ vote is not enough from a leader. We’re asking Senator McConnell to use his position as the Republican Leader to defeat it,” Hoskins wrote on Saturday morning. Hoskins also said the Senate Conservatives Fund will launch an advertising campaign beginning Monday on television and radio directed at McConnell.

Like most members of the Senate Republican caucus, McConnell has not indicated how he will vote on the final bill or its new border security title negotiated by Sens. John Hoeven (R-N.D.) and Bob Corker (R-Tenn.) and the Gang of Eight. However, he has said the underlying Gang of Eight bill has “serious flaws.”