WASHINGTON — (CNN) The marquee fight of the Republican race has finally arrived.

After months of avoiding conflict, Sens. Ted Cruz and Marco Rubio are taking the gloves off over one of the most explosive debates among conservatives: immigration.

For the Cuban-American senators, both of whom are ascendant in polls, rhetorically gifted and financially well-armed, the contrast offers an early chance to shore up their credentials on an issue where they are both vulnerable — and score some points against a possible final rival for the GOP nomination.

At the heart of the battle is the so-called Gang of Eight immigration bill in 2013, which Rubio spearheaded as part of the group. The comprehensive immigration bill written by the senators included a path to citizenship for immigrants living in the U.S. illegally.

Cruz on Thursday went after Rubio on The Laura Ingraham Show, taking his first direct shot at the Florida senator. “He opposed every single one of them. Every single amendment,” Cruz said. “The Gang of Eight voted as a gang against enforcing and securing the border.”

Rubio, on the other hand, has sought to muddy the waters between Cruz’s immigration positions and his own, telling reporters in South Carolina on Thursday that Cruz has been “a supporter of legalizing people that are in this country illegally.”

The bill and Rubio came under heavy fire from conservatives and died in dramatic fashion in the House — and Rubio has sought to distance himself from comprehensive immigration reform since.

Cruz, meanwhile, has positioned himself as tough on illegal immigration and border security — and on Thursday moved to similarly disavow his own prior position on the H-1B visa program.

But Rubio noted that Cruz had his own proposals on that immigration bill in 2013, and drew him close to deflect some of the criticism.

“When the Senate bill was proposed, he proposed legalizing people that were here illegally. He proposed giving them work permits,” Rubio said. “He’s also supported a massive expansion of the green cards. He’s supported a massive expansion of the H-1B program – a 500% increase. So, if you look at it, I don’t think our positions are dramatically different.”

Cruz now no longer supports an increase in the number of visas until the program is reformed, spokeswoman Catherine Frazier said Wednesday.

“Demonstrably false,” Cruz spokesman Rick Tyler said on Thursday in response to Rubio’s comments. “There is no bill, no amendment, no statement, no utterance where Ted Cruz supported a pathway to legalization of people in the country illegally.”

The Rubio campaign continued the attack Thursday when spokesman Joe Pounder tweeted a link to a “FLASHBACK video” of Cruz in a Senate Judiciary Committee where he defends his amendment that gives undocumented immigrants a path to legal status, a chance for the undocumented to come “out of the shadows.” That video was posted by a brand new YouTube account called “Hypo-Cruz.”

Battle for tea party and conservative support

The decision to go after one another directly comes days after the two young senators once again earned plaudits for their debate performances.

Increasingly, the two men — who Rubio said Thursday are friends — are seen as the likely finalists for the GOP nomination. Both have tea party ties, a compelling life story and political experience and are seen as the logical heirs to the voters supporting outsiders Donald Trump and Ben Carson if those campaigns lose momentum.

The seeds of the fight began to sprout Tuesday night, as Trump sparred with Ohio Gov. John Kasich and former Florida Gov. Jeb Bush on his hard-line position of deporting undocumented immigrants living in the U.S.

The debate moderators missed the opportunity to ask Rubio about the argument, but Cruz circled back during his next question.

“The Democrats are laughing, because if Republicans join Democrats as the party of amnesty, we will lose,” Cruz said, without directly calling out his Senate colleague.

But the Texas Republican took his hardline message on immigration to New Hampshire, where he encouraged voters to look at which candidates are “blowing smoke” on a path to citizenship and which are “telling the truth.”

“I like Marco. I respect him,” Cruz told reporters in Kingston, saying the two have had disagreements as part of politics.

Pressed for differences on immigration between him and Rubio, Cruz said simply to look at their records.

“It is not complicated that on the seminal fight over amnesty in Congress, the Gang of Eight bill — that was the brainchild of Chuck Schumer and Barack Obama, would have granted amnesty to 12 million people here illegally — that I stood with the American people and led the fight to defeat it in the United States Congress,” he said.

Cruz has previously insisted on not engaging with Republican opponents until the primary’s “seasons change.” On Wednesday, Cruz began to prosecute that case against those who have left the door open to citizenship, some of whom have used George W. Bush’s rhetoric of “compassionate conservatism” to make their argument.

“There’s nothing compassionate about exonerating the lawlessness and inviting millions of people to come illegal to this country,” Cruz told radio host Tony Perkins earlier Wednesday.

Rubio has repeatedly answered questions about his previous support of immigration reform by blaming an inhospitable climate for its failure. Thursday was no different.

“The American people do not trust the federal government to enforce our immigration laws,” saying the crisis at the border and President’s executive actions have only exacerbated the situation. “I do believe that we have to deal with immigration reform in a serious way and it begins by proving to people that illegal immigration is under control.”

Trump hits Carson

The flashpoint in the GOP race is not limited to Rubio and Cruz.

Rather than wade into the senators’ sparring, Trump on Thursday directed his fire at the surging Carson, tweeting about his openness to some leniency for undocumented immigrants.

“Wow, pres. candidate Ben Carson, who is very weak on illegal Immigration, just said he likes amnesty and a pathway to citizenship,” Trump tweeted.

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