MIDLAND, Texas—Since the voters of this oil-rich district first sent him to the state capitol in 1968, Tom Craddick has coasted through each re-election, swatting away Democratic opponents with relative ease.

But this time, he has had his hands full.

It’s not because his first challenger in eight years, Midland Democrat Bill Dingus, has a realistic chance of unseating him. Rather, it’s that for the first time since he became speaker of the Texas House six years ago, Craddick is having to divert his attention homeward, away from campaigning for another term as speaker.

“This is a really precarious position,” said Ross Ramsey, editor of the political Web site Texas Weekly. “By all accounts, Craddick is taking (the Dingus challenge) seriously. And all of that is time that you’re not spending on the speaker’s race.”

After squashing two bitter attempts to overthrow him last year, Craddick is seeking a fourth term as leader of the divided House. The Republican edge in the chamber has dwindled from 26 to eight seats since he became speaker in 2003. And this Election Day, Democrats have their best chance in more than a decade of further eroding the GOP House majority.

A Democratic majority—or even a small-scale loss of some of Craddick’s allies—could mean the end of Craddick’s six-year reign.