Trump's endorsement looms over Texas special runoff election

The Republican matchup highlights intraparty divides.

The former president also recorded a robo-call that was circulated online that touted Wright as "a great Republican (and) a great woman" who would carry on her husband's politically conservative legacy in Congress. Although Wright's inheritance of her late husband's congressional track record is not an unusual phenomenon in the history of campaign politics, the widow-turned-congressional-hopeful is not yet guaranteed to win outright given Ellzey's fundraising prowess.

Despite not having Trump's endorsement, Ellzey has been able to raise more than double that of Wright. As of July 7, the state congressman raised more than $1.2 million compared to Wright's $454,000, which could have helped him streamline his campaign's voter mobilization efforts ahead of Tuesday's contest.

"Nothing irritates me more than the junk that I have seen in the mailboxes talking about him. If you want to win an election that bad, I don't want you to be my congressman," Perry said at a campaign rally for Ellzey in mid-July.

Following that rally, Club for Growth President David McIntosh issued a statement in which he praised Wright as a "principled conservative" while calling Ellzey a "serial opportunist with a record of missing votes and supporting higher taxes."

But the political back-and-forth could take a backseat to voter engagement given that special elections historically draw far fewer voters to the polls than midterm or general election cycles.

"Susan Wright is still probably the favorite based on the early judgments people made and the Trump endorsement in particular, but I think what makes it unpredictable is that Ellzey is probably a better campaigner than Susan Wright is, and in a very low turnout race -- which this is expected to be -- it's very hard to tell (who will win)," said Cal Jillson, a political science professor at Southern Methodist University who specializes in Texas politics.

Jillson predicts it will be unlikely that Independent and Democratic voters turn out in large enough numbers on what is expected to be a scorching hot day to cast their ballots in opposition to the Trump-backed candidate. Still, the uncertainty of how many voters plan to participate in an off-cycle runoff election looms over the contest.

"You don't know how many people are going to turn out. You don't know who they're going to be, (or) where they're going to be -- the northern part of the district leans toward Wright, the southern part of the district leans toward Ellzey," he said in an interview with ABC News Monday.

Regardless of who comes out on top, the outcome of Tuesday's election signals an inherent victory for congressional Republicans and will further narrow Democrats' majority in the House. The lack of an opposing party member in the running allows Republicans to focus their spending in more competitive contests in the future.

"I look forward to welcoming a new Republican colleague to Congress," National Republican Congressional Committee Chair Tom Emmer said in May following Wright and Ellzey's runoff advancements.