Sen. Lamar AlexanderAndrew (Lamar) Lamar AlexanderSenate GOP urges Trump administration to work closely with Congress on NAFTA GOP lawmakers want Trump to stop bashing Congress The Hill's Morning Report - Sponsored by CVS Health - A pivotal day for House Republicans on immigration MORE's (R-Tenn.) primary challenger, Tennessee state Rep. Joe Carr, picked up another national Tea Party endorsement Thursday, the same day he released a new statewide radio ad. 

The issue at the center of both the Tea Party Patriots’ endorsement and Carr’s new ad is Alexander’s alleged support for “amnesty” for illegal immigrants, a charge Carr has made central to his campaign against the senator.

The Tea Party Patriots endorsement praised Carr as a "genuine leader" in the fight against illegal immigration, criticizing Alexander for being one of the 14 Republicans to vote for the reform bill that passed the Senate last year. 

"Joe wants to preserve American sovereignty; Lamar Alexander's policies are producing a human wave of criminality and disease on the Rio Grande," the conservative group's chairwoman, Jenny Beth Martin, said in a statement.

Carr's radio ad also hits Alexander on his support for the immigration measure, charging that the senator "voted with Democrats to pass a bill creating a border crisis by giving amnesty to 11 million illegal aliens." 

The 60-second spot attacks Alexander for refusing to accept Carr's invitation to debate the issues, saying the senator would rather have voters read his website. 

"Lamar's website doesn't tell you a lot of things," Carr says in the radio ad. 

Carr led the charge for stricter immigration laws in the Tennessee Legislature and believes the focus on immigration reform could pay off for him in the Aug. 7 primary election.

Despite the attacks on immigration, Alexander remains poised for reelection, with every survey of the race showing him ahead by double-digit margins.

And the senator continues to draw conservative support of his own, boasting endorsements from former Arkansas Gov. Mike Huckabee and former House Speaker Newt Gingrich (Ga.).