Rubio caught fellow members of the Senate’s gang off guard Tuesday when he voiced support for an amendment sponsored by Sen. Jeff Sessions (R-Ala.), the bill’s most outspoken opponent, to dramatically strengthen the system for tracking entry and exit visas. The Sessions measure was defeated by the Judiciary Committee on Tuesday. Rubio does not sit on the panel.

“He needs this bill to succeed as much as Democrats do. If this bill goes down, he goes down with it,” warned a senior Democratic aide. “Rubio is overplaying his hand if he thinks we’ll go along with anything.”

Democrats found Rubio’s support for the proposal egregious because Sessions is an avowed opponent of the bipartisan measure. The amendment would have required the implementation of a system to track biometric data, which Democratic senators on the Gang of Eight say could add as much as $25 billion to the bill’s price tag.

Two Gang of Eight sources characterized Rubio’s support for the Sessions amendment as a “mistake.”