Updated 12:05 a.m.

Unofficial election returns indicate incumbent state Sen. Jeff Wentworth will be heading to a runoff July 31 against Donna Campbell.

Wentworth garnered 35.77 percent of the vote, followed by Donna Campbell and Elizabeth Ames Jones with 33.68 percent and 30.53 percent, respectively.

Campbell ultimately came from behind in what had mostly been a dead heat between Wentworth and Jones.

"Campbell is in second place and Ames is in third, which is a surprise to many," Wentworth said.

The senator said it was only the second time in 20 years he was facing a runoff—the first being in 1992, when he first won his state senate seat.

Posted 10:35 p.m. In preliminary early voting results, incumbent state Sen. Jeff Wentworth has a slight lead over challengers Elizabeth Ames Jones and Donna Campbell, who are virtually tied for second place in the Republican primary race for Texas Senate District 25.

With 140 out of 316 precincts reporting, Wentworth, who has served in the state senate since 1993, is leading with 35.95 percent of the vote, while Campbell and Jones have 32.50 percent and 31.54 percent, respectively.

District 25 spans from San Antonio to Austin and includes Bexar, Comal, Guadalupe, Hays, Kendall and Travis counties.

Campbell, a physician from New Braunfels, ran for Texas' 25th Congressional seat in 2010 against eight-term Democratic incumbent Lloyd Doggett. She garnered 45 percent of the vote in that race.

Jones represented Bexar County in the state House of Representatives from 2000 until 2004, when she was appointed to the railroad commission. She stepped down in February to devote more time toward her campaign for state senate.

The race between Wentworth and Jones has been especially contentious as the senator filed a defamation lawsuit against the former railroad commissioner regarding campaign spending accusations. Jones has publicly dismissed the suit as unmerited.

John Courage, a teacher from San Antonio, is running unopposed in the District 25 Democratic primary.