Although he didn't use the word "electable," which to many activists connotes selling out conservative values, Rubio's focus was squarely on what he would bring to the national ticket in November.

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"If you nominate me, we won't just unify the Republican Party, we will grow the conservative movement," he said. "Because I will able to go to people all across this country who have not voted for us before and I will be able to say to them: 'I grew up the way you now live.'"

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Asked afterward whether Cruz and Trump can beat Hillary Clinton, Rubio responded: "I believe I give us the best chance."

Ernst made clear that she was not endorsing Rubio — or anyone, for that matter — but she called him her "good friend" and praised his approach to national security policy.

"I'm going to introduce someone who is very near and dear to my heart," Ernst said in her opening remarks. Several of Rubio's top strategists worked for Ernst during her 2014 Senate campaign, and Rubio supported her bid.

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"I want someone who will destroy ISIS," said Ernst, an Iraq war veteran, using an acronym for the Islamic State terrorist group.

Rubio mentioned Ernst's name several times in his remarks. He warned, at length, of what he sees as the dangers of losing to Clinton in the general election, arguing that she would uphold President Obama's "radical executive orders" and that under her Obamacare "will be here to stay," among other things.

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"If Hillary Clinton is elected president of the United States, the consequences will be extraordinary," Rubio said.

"We cannot lose this election," he said at one point, before repeating himself for emphasis.

Rubio said Republicans need "to nominate someone who understands what people are going through, who can grow this party because we understand what people are facing, and I do." He said Clinton would not be able to lecture him about student loans, raising children in the current economy and other issues because he has experienced those things firsthand.