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.

Sen. Mark Kirk (Photo by Brian Kersey/Getty Images) Sen. Mark Kirk (Photo by Brian Kersey/Getty Images)

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.

"I support a two-step immigration reform that first secures our southern border and then creates a tough but fair path to citizenship for immigrants living illegally in the United States," Kirk said in the statement. "I have always believed that is the recipe for bipartisan consensus on this issue. Once we restore the public’s trust in our government’s ability to control the border, we can move forward with other reforms."

Specifically, Kirk wants an amendment by Sen. John Cornyn (R-Texas) -- or something similar -- that would scrap the border security language in the bill for tougher measures. The amendment would only trigger a path to citizenship if and when certain border security requirements are satisfied.

The Cornyn amendment has the support of "Gang of Eight" member Sen. Marco Rubio (R-Fla.) and Senate Minority Leader Mitch McConnell (R-Ky.), but it has been described by Democrats as a "poison pill" that would effectively kill the legislation.

Kirk bristled at that characterization in his statement.

"If requiring real border security with verifiable metrics and independent certifications is a 'poison pill' for the Democratic leadership, then I fear we are setting a course for division and partisanship," Kirk said. "My votes yesterday were a demonstration of this great disappointment."

Kirk's stance is particularly interesting because he has been a go-to Republican for Democrats seeking bipartisan support on their issues. Kirk backed the Democrats' gun control legislation and has also come out in favor of gay marriage in recent months -- making him one of just two Republican senators to do so.