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Santorum's endorsement comes after Sens. Jon Kyl and John McCainJohn Sidney McCainTrump's plan to claw back spending hits wall in Congress Defense bill moves forward with lawmakers thinking about McCain How House Republicans scrambled the Russia probe MORE, both Arizona Republicans, endorsed Flake in the increasingly contentious primary. And the Club for Growth, the prominent conservative organization, also pledged to spend half a million dollars on advertisements supporting the Arizona lawmaker.

Cardon now hopes that Santorum can rally Tea Party conservatives in the state. A poll released late last month by Magellan Strategies found Flake with a 22-point lead in the primary battle, despite Cardon's million-dollar ad blitz against the six-term congressman.