But the Corker-Hoeven push for allocating thousands of additional border agents appeared to change few minds within the Republican conference.

“I didn’t see it move many people. It’s not going to get me to vote for it,” said a lawmaker who attended a meeting at which Republican senators discussed the proposal. “I’m not going to vote for an immigration bill that doesn’t have a metric for securing the border.”

There is now growing uncertainty about whether Rubio — in the face of sustained criticism on the right — will end up voting for the Gang of Eight’s bill.

Rubio has declined to outline his plans, unlike other members of the Gang of Eight, such as Sen. Lindsey Graham (R-S.C.), who have pledged their support.