A boiling primary battle in Texas headed to a runoff early Wednesday as two Republicans running for U.S. Senate failed to reach the 50% threshold to clinch the GOP nomination.

Though Lt. Gov. David Dewhurst led the crowded field of candidates, he narrowly failed to cross the 50% mark required to secure the nomination and avoid a July 31 runoff.

Dewhurst's challenger in the runoff will be tea party favorite Ted Cruz, a former solicitor general with strong support from national groups and high-profile conservative leaders.

With 92% of precincts counted, Dewhurst held 48% of the vote to 30% for Cruz.

Other contenders in Tuesday's primary included former Dallas Mayor Tom Leppert and ESPN Broadcaster Craig James. Neither candidate will forge ahead to compete in the runoff.

The Texas race is particularly important because the balance of power in the U.S. Senate could shift in 2012. It is also another test in the staying power and influence of tea party groups and their political strategy to focus more on congressional races than on the presidential campaign.