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In the final Republican presidential debate of 2015, no subplot mattered more than the growing feud between Sens. Ted Cruz of Texas and Marco Rubio of Florida.

Senator Cruz, surging in the polls, is an anti-GOP-establishment outsider 鈥� like front-runner Donald Trump 鈥� but with stronger conservative bona fides than Mr. Trump and significant appeal among evangelicals.

Senator Rubio, now polling third nationally, represents a kinder, gentler face of Republicanism, one with a more collegial relationship with fellow senators than Cruz and more potential to attract swing voters in the general election.

With Trump鈥檚 national lead continuing to grow, to the chagrin of party leaders, the battle to become the main alternative to Trump is more pitched than ever as the kickoff Iowa caucuses on Feb. 1 draw closer. 聽

But the battle didn鈥檛 play out Tuesday night in attacks on Trump. Indeed, Cruz and Rubio both went easy on the flamboyant billionaire, in a likely effort to woo his supporters should they cool to his unorthodox campaign persona and style. Instead, the two Cuban-American senators went after each other in Las Vegas on national security and immigration, the evening鈥檚 dominant themes in the wake of terror attacks in Paris and San Bernardino, Calif. Here鈥檚 how those exchanges played out:

NSA phone record surveillance. Rubio has been going after Cruz lately over his vote for the USA Freedom Act, which imposed new limits on the collection of phone metadata by US intelligence agencies in an effort to protect civil liberties.

鈥淚 promise you, the next time there is attack on 鈥� 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?鈥� Rubio said Tuesday. 鈥淎nd 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.鈥�

Cruz defended the legislation as expanding the potential universe of records the government has access to. What he didn鈥檛 say is that the government now must get a court order to access them, which defenders say does not represent a serious hurdle.

Defense spending and approach to Islamic State. Rubio also went after Cruz for voting multiple times against legislation that authorizes defense spending.

鈥淵ou can鈥檛 carpet bomb ISIS if you don鈥檛 have planes and bombs to attack them with,鈥� Rubio charged, referring to the so-called Islamic State. 鈥淭hey cannot be defeated through air forces.鈥�

Cruz has promised to 鈥渃arpet bomb鈥� ISIS with air strikes, and is less enamored of sending US ground troops to Syria and Iraq than is Rubio. In the debate, he defended his 鈥渘o鈥� votes against the defense authorization act as fulfillment of a promise to oppose the federal government鈥檚 authority to detain US citizens without due process.

Cruz also tried to lash Rubio to the policies of President Obama and former Secretary of State Hillary Clinton, the front-runner for the Democratic nomination.

"We need to focus on killing the bad guys, not getting stuck in Middle Eastern civil wars," Cruz said.

Immigration. For Rubio, this issue is the most fraught, as it gives some conservatives pause over his candidacy. As a new senator, he co-authored comprehensive immigration reform legislation, only to renounce the bill 鈥� which included a path to citizenship for undocumented immigrants.

Rubio has sought to paint Cruz as favoring 鈥渓egalization鈥� of the undocumented, over his support for big increases in the caps on green cards and the number of visas for high-tech workers. Cruz has since backed off that position, and in the debate, he fought back.

鈥淚 led the fight against [Rubio鈥檚] legalization and amnesty,鈥� Cruz said, adding that 鈥渢o suggest our record's the same is like suggesting the fireman and the arsonist have the same record because they are both at the scene of the fire."

In debate post-mortems, both senators were deemed to have had strong performances, and their duel is likely to intensify in the weeks ahead.

That Cruz and Rubio are both Cuban-American first-term senators in their mid-40s may be an accident of history, but it matters to a Republican Party anxious to showcase diversity. Whether either could make major inroads into the Hispanic vote is an open question, particularly for Cruz, who doesn鈥檛 speak Spanish and who only uses his heritage to highlight his Cuban-born father鈥檚 journey to freedom in the US.

Rubio, on the other hand, is fluent in Spanish and has made his Cuban-immigrant parents鈥� humble lives as service-workers a central part of his optimistic view of the American dream.

If any of the Republican candidates are capable of channeling Ronald Reagan, whose sunny demeanor helped him to the presidency twice, it may be Rubio. Likability is a key ingredient to presidential campaign success, and if support for Trump begins to wane, one can鈥檛 assume that his voters go to Cruz.

鈥淭oo often [Cruz鈥檚] message seems negative, and I think that鈥檚 a challenge for him,鈥� says Henry Barbour, Republican national committeeman from Mississippi, speaking before Tuesday鈥檚 debate. 鈥淭he key is to come across as the guy with the positive agenda 鈥� the Ronald Reagan of the group. And everybody wants to be the Ronald Reagan.鈥�