Heritage Action, the political arm of the conservative Heritage Foundation, came out Friday against Sen. John Cornyn's (R-Tex.) border security amendment, saying its passage would give false legitimacy to the Gang of Eight's immigration bill.

In a statement, Heritage Action CEO Michael Needham said it "will oppose amendments that, if adopted, will serve as political cover for those senators seeking to justify their support for amnesty.”

Senate conservatives have been pushing for the Cornyn amendment, which requires certain border security requirements to be satisfied before it would trigger a path to permanent legal status or citizenship.

The amendment has been cast as a way to satisfy Republican concerns about the bill and make it more passable in the GOP-controlled House. Senate Majority Leader Mitch McConnell (R-Ky.) and Sen. Mark Kirk (R-Ill.) have both suggested the Cornyn amendment's passage may win their votes for the larger package, and Gang of Eight member Sen. Marco Rubio (R-Fla.) has also said he wants more border security in the final bill.

Democratic leaders, meanwhile, have suggested the Cornyn amendment is a "poison pill" that would lead Democrats to oppose the larger bill.

In response to Heritage's decision to come out against the measure, Cornyn's office noted that critics of the immigration package, including Sens. Mike Lee (R-Utah), Ted Cruz (R-Texas), Charles Grassley (R-Iowa) and Jeff Sessions (R-Ala.) voted for it in committee.

"Sen. Cornyn is pleased with the support he has received and looks forward to calling up his amendment," Cornyn spokesman Drew Brandewie said in a statement.

Heritage Action notes that immigrants are given temporary legal status before the border security conditions are met, which it says constitutes amnesty.

It will also score the vote as a key vote on its congressional scorecard, which rates members according to their conservatism.

Heritage, which is headed up by former senator Jim DeMint (R-S.C.), has staked out some very conservative territory in opposition to the immigration bill.

A Heritage Foundation study last month suggested that the bill could cost more than $6 trillion. But the study was widely panned by Republican supporters of comprehensive immigration, and one of its author's later resigned after it was revealed that he had written for a Web site with nationalistic leanings.

Updated at 4:12 p.m. to include comment from Cornyn's office.