Sen. Mark Kirk (R-Ill.), one of the leading GOP moderates in the Senate, said Wednesday that he will withhold his support from a bipartisan immigration bill unless it adopts a stronger border security component.

Kirk previously suggested he might be willing to vote for the bill, saying in recent weeks that the bill had a “bright future.” Nonetheless, he was one of just 15 Republicans to vote Tuesday against proceeding with debating it, joining members of the more conservative wing of the party.Kirk said in a statement Wednesday that more border security must be in place before he will agree to a path to citizenship for illegal immigrants.