Club for Growth launches $1.5M ad buy against Dewhurst in Texas

The group, which spent $2.5 million in late May to make sure Cruz made the runoff, promises to be on the air between now and Election Day, July 31. Their new ad blasts Dewhurst as a "big-spending, tax-raising" moderate and touts Cruz's conservative credentials.

Dewhurst won't be outspent — he has a personal fortune that he used in the first round of voting. But the Club's commitment to help Cruz stay competitive on the airwaves could be a big boost to his campaign.

Both Cruz and Dewhurst have touted internal polls in recent days that show them with high-single-digit leads in the race, a sign that the runoff election might be a close one.