The intensifying rivalry between Ted Cruz and Marco Rubio grew nastier Tuesday when a small super PAC supporting Cruz hit Rubio hard with a new ad accusing him of trying to concoct an "amnesty scheme" as part of a bipartisan group of senators pushing comprehensive immigration reform.

The ad, from Courageous Conservatives PAC, marks the latest broadside in a quickly escalating battle between the two Republican senators and presidential aspirants that has erupted in recent days.

"Marco Rubio teamed up with John McCain, Chuck Schumer, Lindsey Graham and Bob Menendez to push his 'Gang of Eight' amnesty scheme," says the narrator of the ad, highlighting two centrist Republicans and two Democrats Rubio worked with in 2013 to pass a sweeping immigration reform bill that included a path to citizenship for undocumented immigrants. The ad claims that Rubio tried to shut down Cruz's efforts to make the bill more conservative.

Cruz, a Texas senator, voted against the bill and the ad highlights his opposition to it.

After the bill passed the Senate, it was killed in the House by conservatives. Rubio, a Florida senator, abandoned his push for comprehensive reform and now favors a piecemeal approach that begins with border security and enforcing current laws before eventually opening up a path to citizenship.

It wasn't immediately clear whether the ad was running online only or on television as well, though it included disclosures at the end typically associated with TV ads. Nor was it clear how much money was behind the ad.

Rick Shaftan, a consultant for Courageous Conservatives PAC said the ad is running on radio in Iowa, backed by $15,000 including production costs. The video component is only running online.

The organization is one of several pro-Cruz super PACs. The group released a radio ad this month hitting Rubio for accomplishing little during his time in the Senate.

Cruz and Rubio have been sparring over immigration in recent days. Rubio and his backers have argued that the two have comparable records when it comes to offering legal status to undocumented immigrants, pointing to an amendment Cruz pushed that would have granted work permits for undocumented immigrants.

Cruz's campaign has countered that the amendment was a gesture intended to highlight the "hypocrisy" of Democrats' focus on a path to citizenship. Overall, Cruz is seeking to run to Rubio's right on immigration, casting himself as a harder-line opponent of illegal immigration.

On Monday, Rubio went hard after Cruz on national security, accusing him of voting to "weaken" surveillance efforts with his support for the USA Freedom Act of 2015.