Republican presidential candidate Marco RubioMarco Antonio RubioColbert: Students taking action on gun violence 'give me hope' Lawmakers feel pressure on guns Florida lawmaker's aide fired after claiming shooting survivors were 'actors' MORE is arguing he's the party's best shot at defeating Hillary ClintonHillary Diane Rodham ClintonWoman behind pro-Trump Facebook page denies being influenced by Russians Trump: CNN, MSNBC 'got scammed' into covering Russian-organized rally Pennsylvania Democrats set to win big with new district map MORE in his closing TV ad for Iowa, days before the state's caucuses.

"America is exceptional. Every generation before us met the great challenges of their time. But after seven long years of this president, we feel our country slipping away," Rubio says in the ad released Thursday. "This election is about defeating Hillary Clinton and about saving what makes America unique."

"I approved this message, and I am asking for your vote so we can leave our children what our parents left us: the single greatest nation in all mankind," he adds.

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A statement from the Rubio campaign referred to the ad, titled "Defeating Hillary," as their "closing TV ad in Iowa."

But before Rubio faces Clinton, the Democratric presidential front-runner, he has to make it out of the crowded nomination field. He sits in third place in the RealClearPolitics average of recent polls in the state and regularly uses that contrast with Clinton as he makes his pitch to voters. 

He repeats asserts that Clinton is "disqualified" to serve as president because of the controversy around her use of a private email server while secretary of State. 

And during a campaign stop on Wednesday in West Des Moines, he framed the millions of dollars spent on negative ads against him as counterproductive to beating Clinton.

"This money was not raised $50 at a time, these are big checks from people who agree we need to elect the next person in line," he said. 

"We've done that before. You know what happens when we do that? We lose. You know what happens when we lose now? You wake up the first Wednesday in November to the news that Hillary Clinton is now president of the United States. That would be a disaster. That's why I ran for president — we will win if I am the nominee."