LAMONI, Iowa — Ted Cruz and his campaign are offering their harshest and most extensive criticism yet of Marco Rubio since the U.S. Senate colleagues began clashing over immigration and national security issues.

Speaking with reporters before a town hall here, Texas' junior senator repeatedly pushed back on a TV ad airing in Iowa that suggests Cruz "voted to weaken America's ability to identify and hunt down terrorists." The spot was produced by an outside group whose head supports Rubio, the Florida senator vying with Cruz for the GOP presidential nomination.

"I think Sen. Rubio’s campaign is very, very dismayed that conservatives [are] coming together behind our campaign and is attempting to mislead the voters in an effort to slow that down,” Cruz said. "I don’t believe that’s going to be successful.” 

Cruz's remarks came hours after his campaign released a video featuring one of his supporters in Congress, U.S. Rep. Jim Bridenstine of Oklahoma, criticizing Rubio for his involvement in the Gang of Eight immigration reform bill two years ago. That legislation, Bridenstine said, "would have expanded the U.S. Refugee Resettlement Program and given Obama authority to admit even more Syrian refugees." Cruz echoed that criticism Saturday, saying it has become "abundantly clear" following the recent Paris terrorist attacks that the proposal was "significantly harmful to our national security." 

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The anti-Cruz ad, which focuses on his vote in favor of the USA Freedom Act in June, is an affront to people like Bridenstine, Cruz said, calling the congressman a veteran and war hero. 

"The Rubio attack ad is impugning his patriotism," Cruz told reporters. "It’s suggesting that conservatives like [Utah Republican U.S. Sen.] Mike Lee, like me, like the NRA, somehow acted to undermine our national security. That’s about as ridiculous a proposition as anyone can imagine."

The USA Freedom Act placed new limits on national intelligence programs, legislation that Rubio opposed and is seeking to capitalize on in the wake of the recent Paris terrorist attacks. Cruz defended his support of the bill Saturday, saying it not only ended bulk collection of data on Americans' phone records but also "expanded our tools to target the bad guys, to target terrorists."

“I imagine Sen. Rubio’s PAC is trying to respond to the criticism that is receiving that he was not willing to protect the Fourth Amendment privacy rights of law-abiding citizens, and they are attempting to do so by attacking those of us who were," Cruz said. 

Asked for comment on Cruz’s remarks, a Rubio spokesman pointed to a statement the Floridian released earlier Saturday.  

"When the clock strikes midnight, Americans will be less safe because of a presidential leadership void and gross misinformation that combined to kill critical intelligence programs at a time of rising terrorist threats to the United States and our allies,” Rubio said in the statement. "By abandoning these programs and enacting the USA Freedom Act, President Obama and his allies in Congress are unilaterally disarming the U.S. in the fight against terrorism and take (sic) away a valuable early warning tool from people whose sole duty is to keep us safe."

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Cruz repeatedly referred to the group running the ad, American Encore, as “Rubio’s PAC.” American Encore is run by Sean Noble, a former operative in the Koch brothers network who personally backs Rubio. 

Asked if thought American Encore and the Rubio campaign were teaming up, Cruz said, “None of this is rocket science ... [Rubio] goes on national television, he makes this bogus attack and within days magically the super PAC echoes the same exact attack."

Cruz's comments on Rubio came near the end of the first day of three-day, 14-stop swing through early voting Iowa. He is being accompanied by U.S. Rep. Louie Gohmert, one of six GOP members of Texas' congressional delegation who have endorsed Cruz for president.