By James Oliphant and Emily Stephenson
LAS VEGAS, Nevada (Reuters) - 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.
As such, they engaged in an arm-wrestling contest for most of the evening, sparring on Middle Eastern policy, national security and immigration.
"The Cruz-Rubio battle is now a defining dynamic in this race," said Ford O'Connell, a Republican strategist.
Both largely left Trump alone - and in fact, when Cruz was invited by debate moderators to attack the real estate mogul, he demurred.
But Cruz had no such restraint when it came to Rubio. Among other criticisms, he accused him 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."
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.
A win by Cruz in Iowa could severely damage Trump's bid, as the real estate mogul's political message is largely grounded in his current dominance of opinion polls. It could also hand Cruz the kind of momentum that could derail Rubio's bid to be the candidate around whom anti-Trump voters rally.
"They're (both) trying to reach as wide a swath of non-Trump voters as possible," O'Connell said.
They are, however, doing that in different ways.
In Las Vegas on Tuesday evening, Rubio articulated a muscular national security outlook, both abroad and at home, defending his support for U.S intervention in Libya in 2011, calling for an increase in the number of U.S. ground troops in Syria and Iraq in the struggle against Islamic State, ramping up military spending, and intensifying domestic surveillance programs.
"There are serious policy differences between Marco and Senator Cruz, especially on defense issues, and we've been pointing those out for the last couple of weeks," Rubio's spokesman, Alex Conant, said following the debate.
Cruz, conversely, advocated a more restrained foreign policy, arguing that a bombing campaign against Islamic State would suffice. He contended that the U.S. government had been allowed to collect too much data on Americans in the name of foiling terror attacks.
"This is an area where the American people deserve to see contrast," said Chad Sweet, Cruz's campaign manager.
In that sense, the two senators were addressing disparate elements of the Republican electorate in Iowa and elsewhere, said Matt Strawn, a former chairman of the state's Republican Party, who said, because of that, there was "no real blood spilled" between the two.
"Cruz's comments on (phone call) metadata help him with a significant share of Iowa's libertarian-oriented vote, while Rubio's responses are exactly what traditional national security conservatives in Iowa want to hear," Strawn said. "Little happened to alter the dynamic of the race as we approach Christmas."
But both men's gambits on Tuesday may have had an unintended consequence.
Republican strategists told Reuters that Cruz and Rubio's relentless focus on each other may have opened a door for New Jersey Governor Chris Christie to gain a stronger foothold in the race by allowing him to contrast their relatively scant time in the Senate with his five years as a state executive.
At one point Tuesday, the two-term governor mocked them as they groused at each other about their Senate records.
"If your eyes are glazing over like mine, this is what it's like to be on the floor of the United States Senate," Christie said. "Endless debates about how many angels on the head of a pin from people who've never had to make a consequential decision in an executive position."