OMAHA, Neb. — The Democratic Party’s big tent has shrunk a little with the defeat of Heath Mello, whose failed bid for Omaha mayor exposed a rift in the party on abortion and the viability of the progressive wing in middle America.

A pro-life Democrat, Mr. Mello lost his bid Tuesday to unseat incumbent Republican Mayor Jean Stothert in a race that sparked a national debate on whether Democrats who stray from the party’s pro-choice platform are still welcome.

Ms. Stothert, 63, won re-election by 53 percent to 47 percent in a race that was pitched by Democrats as a referendum on President Trump, who won red-state Nebraska but lost left-leaning Omaha to Democrat Hillary Clinton in November.

For Democrats, the defeat comes with the party banking on a midterm surge next year in order to take advantage of Mr. Trump’s lukewarm poll numbers, even though the party has been unable to capitalize on its opportunities.

Democrats also came up short in a special election for a House seat in Kansas and failed to clinch an open House seat in Georgia.

Mr. Mello, 37, raised the profile of the local race by snaring the endorsement of Sen. Bernard Sanders of Vermont, who appeared at a high-profile event in Omaha with Democratic National Committee Deputy Chairman Keith Ellison.

But the Mello campaign became mired in abortion politics. On April 20, the same day as the Sanders rally, the Daily Kos withdrew its endorsement over Mr. Mello’s pro-life record.

That was also the day that NARAL Pro-Choice America blasted the Democratic National Committee as “politically stupid” for backing Mr. Mello.

Kristen Day, who heads Democrats for Life of America, accused the pro-choice movement of putting its issue ahead of the good of the party.

“The abortion rights movement shouldn’t have been involved in that race at all,” said Ms. Day. “If they cared at all about the party, they would have left Health Mello to run and stay true to his conscience. He may have been able to win. Instead, the abortion rights activists are trying to keep middle America in Republican hands.”

Stothert spokesman Dave Boomer said the Sanders endorsement, along with the party’s mixed messages on abortion, wound up backfiring on the Mello campaign.

Mr. Sanders “really inflamed our base. It pushed people to us,” Mr. Boomer said. “And I think the abortion thing really hurt Mello because the messages that came out of that party were like 180 degrees from one another. Some people were saying, ‘Oh, it shows big tent,’ and others were, ‘Oh, no, if you’re not pro-choice, you should not have the support of the Democratic Party.’”

Randall Adkins, University of Nebraska at Omaha political science professor, said the episode threw the Mello campaign off message for a week in a runoff election that lasted five weeks.

“I don’t think [the abortion issue] had a direct impact. It’s that, combined with the visit of Bernie Sanders, that takes Heath Mello off of his core message for about a week,” said Mr. Adkins. “It really hurt because Bernie came in, the abortion issue got raised, and they’re not local issues.”

Mr. Mello, who had sponsored and backed bills restricting abortion access in the state Legislature, later said that while he was personally opposed to abortion, he would “never do anything to restrict access to reproductive care.”

To Democrats on the coasts, Mr. Mello may have seemed like an outlier, but not in Omaha, which is split roughly evenly among Democrats, Republicans and nonpartisans and has a large Catholic population.

“There are still a lot of Democrats out there that I think are pro-life,” said Mr. Adkins.

On the ground, the campaign focused for the most part on municipal issues such as potholes, public safety, taxes and a proposed streetcar project, although Mr. Mello continued to play the Trump card until the end, blasting the president for the “travel ban” and Obamacare repeal.

“America needs a mayor who stands up to Trump,” the Mello campaign said in a May 4 press release.

Jane Kleeb, an anti-Keystone XL activist who chairs the Nebraska Democratic Party, described Mr. Mello’s defeat as a blow to the party.

“We’re in a position where we have to build up the party from the grass roots on up, and it is going to take us several years,” Ms. Kleeb told the Omaha World-Herald. “So this is a serious and hurtful loss. Heath Mello was one of the best candidates we have on our bench, but it’s always forward, never back for us at this point.”

Ms. Stothert wasn’t one of the Trump faithful — she wrote in Sen. John McCain of Arizona on her presidential ballot. But after the April 4 primary, she reached out to Trump voters by forming an alliance with Taylor Royal.

Mr. Royal, a 27-year-old political newcomer, placed a surprising third in the jungle primary with 11 percent of the vote after running as a Trump-backing fiscal conservative who wanted to bring an NFL team to Omaha.

After the primary, Mr. Royal endorsed Ms. Stothert, cut a radio ad on her behalf and appeared with her frequently on the campaign trail.

“We had him at all our events. At our wrap-up rally, he introduced her,” said Mr. Boomer. “We needed to solidify the conservatives, so Taylor helped with that. But so did Bernie Sanders.”

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