While many viewers were fixed on the ongoing political spectacle that is Donald Trump during Tuesday's Republican presidential debate, Ted Cruz and Marco Rubio only had eyes for each other.

The two first-term senators – one from Texas and the other from Florida, both the 44-year-old sons of Cuban fathers and both rising conservative stars in the party – made it evidently clear that they see the other as the primary obstacle to securing the nomination if Trump, the current front-runner, falters.

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Among other criticisms, Cruz accused Rubio of being soft on immigration policy because he helped craft a comprehensive reform measure in the Senate.

“He was fighting to grant amnesty and not to secure the border. I was fighting to secure the border,” Cruz said.

For his part, Rubio charged that Cruz had helped make the United States more vulnerable to a terror attack by supporting a bill that scaled back the reach of U.S. surveillance programs.

“The next time there is attack on – an attack on this country, the first thing people are going to want to know is, why didn't we know about it and why didn't we stop it?” Rubio said. “And the answer better not be because we didn't have access to records or information that would have allowed us to identify these killers before they attacked.”

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The public spat has been brewing for weeks, with each campaign regularly criticizing the other in the media as Cruz has surged. A recent opinion poll by the Des Moines Register had Cruz leading Trump in Iowa, which holds the nation’s first nominating contest on Feb. 1, 2016. Trump, however, still leads in national polls.