Marco Rubio and Ted Cruz are starting to take shots at each other

Sen. Ted Cruz (R-Texas) and Sen. Marco Rubio (R-Florida), two increasingly surging members of the Republican presidential field, are starting to snipe at one another.

Their jostling on Wednesday and Thursday came over perhaps the most contentious issue in the GOP primary: immigration. 

While being careful to avoid criticizing Rubio directly by name during an interview with Laura Ingraham on Thursday, Cruz knocked the Florida senator and several other Republicans for their support of the 2013 immigration-reform package that stalled in Congress.

"Talk is cheap," Cruz said.

The Texas senator pointed out that unlike Rubio, who now says that border security measures need to be implemented before a pathway to legal status can be established for immigrants living in the country unlawfully, Cruz believed in securing the border first all along.

"The argument that we need to secure the border first is an argument that I was making over, and over, and over again," Cruz said. When asked whether Rubio supported Cruz's border security amendments, Cruz said Rubio "opposed every single one of them."

"As a voter, when politicians are saying the exact opposite of what they have done in office, I treat that with a healthy degree of skepticism," he added.

Part of Cruz's hesitation to attack Rubio directly is keeping with his early campaign pledge that he would not attack fellow Republicans during the campaign.

For his part, Rubio appeared to have a counter-punch at the ready. He defended his own record by claiming that Cruz actually supported some of his 2013 immigration proposals, including an expansion of temporary visas and green cards, which in some cases can create a path to citizenship.

"Ted is a supporter of legalizing people that are in this country illegally," Rubio said, according to CNN. "In fact, when the Senate bill was proposed he proposed legalizing people that were here illegally - he proposed giving them work permits. 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."

"If you look at it, I don't think our positions are dramatically different."

Rick Tyler, a Cruz spokesman, responded on Twitter that Rubio's comments were "demonstrably false."

Cruz isn't alone in questioning Rubio's seemingly shifting position on immigration.

Earlier this month, Sen. Rand Paul (R-Kentucky) said Rubio's attempt to emphasize increased border security measures and distance himself from the 2013 effort didn't adequately acknowledge his role in crafting the bill.

"He was a co-author of the bill. It was a Rubio bill. It was a Rubio-Schumer bill," Paul said, referring to Sen. Chuck Schumer (D-New York). "So, he does have to explain it. I think it will be a big part of things."

And in multiple interviews on Wednesday, Rubio was forced to defend his comments that he would end President Barack Obama's program allowing immigrants brought to the US as children to qualify for temporary work permits, despite saying previously that ending the program wouldn't be fair without an appropriate replacement.