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Sen. Ted Cruz on Thursday moved to blunt Sen. Marco Rubio's momentum in the race for the Republican presidential nomination, launching an aggressive first strike on immigration.

The Texan attacked Rubio for supporting the granting of a pathway to U.S. residency for illegal immigrants in his role as key negotiator of the bipartisan immigration overhaul that cleared the Senate two years ago. The broadside suggests that Cruz now views the Floridian as a distinct political threat to his White House ambitions. It also marked an escalation and possible strategic shift for Cruz, who had shied away from directly attacking his primary opponents.

"I'm confident that Sen. Cruz's position is the winning position, and that if we nominate someone who is pro-amnesty, the Republicans will lose to Secretary [Hillary] Clinton," Cruz campaign spokesman Rick Tyler said, in an interview with the Washington Examiner.

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"This election is going to be about the future," countered Rubio spokesman Alex Conant. "Marco will win because he best understands the new challenges and opportunities of the 21st Century."

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