LOUISVILLE, Ky.—Democrats Wednesday launched the second phase of their fight to capture a long-held Republican House seat in Georgia, but the party’s attempts to unify remain hindered by lingering internal divisions.
First-time candidate Jon Ossoff, who had raised more than $8 million in a matter of months from Democrats across the country, garnered 48.1% of the vote in a crowded open primary Tuesday, just shy of the 50% threshold needed to capture the seat outright.
Now, he faces a June showdown with Republican Karen Handel, a former secretary of state whose 19.8% of the vote topped the field of 11 GOP candidates, in an election to fill the seat vacated by newly appointed Health and Human Services Secretary Tom Price.
While Mr. Ossoff’s candidacy is drawing national attention and has been a rallying point for many grass-roots activists, he has run as a traditional Democrat without adopting the fiery tone powering the liberal resistance to President Donald Trump across the country.
Vermont Sen. Bernie Sanders, in an interview Tuesday in Louisville, Ky., said he didn’t know much about Mr. Ossoff, a 30-year-old former House staffer. Mr. Sanders said he isn’t prepared to back Democrats just because of a party label.
“If you run as a Democrat, you’re a Democrat,” he said. “Some Democrats are progressive and some Democrats are not.”
Asked if Mr. Ossoff is a progressive, Mr. Sanders, an independent who challenged Hillary Clinton in the 2016 presidential primary, demurred. “I don’t know,” he said.
Mr. Ossoff’s campaign didn’t respond to requests for comment.
The Georgia special election is just one stop on the Democrats’ course for a comeback after losing the White House in the November election and it isn’t an easy path.
They need to take 24 seats to regain the House majority. The next contests are in Montana and South Carolina, which are also GOP strongholds.
Democratic National Committee Chairman Tom Perez took the strength of Mr. Ossoff’s campaign as the latest signal that Democrats are making headway.
“They’ve pulled out the heavy artillery, they have Donald Trump making robocalls,” Mr. Perez said. “My main message from this is swing the bat, swing it early, swing it often and swing it with your best shot.”
Mr. Ossoff did draw more votes on Tuesday than the Democratic candidate in the 2014 midterm. About 92,300 people backed him, compared with 71,400 who voted Democratic four years ago.
Still, the runoff with Ms. Handel will be daunting, given the partisan leaning of the district and that Republican votes won’t be splintered by multiple candidates. The open primary on Tuesday featured 18 candidates, and most were Republicans.
“Republicans are united,” she told CNN Wednesday. “We know this is an important race and it is going to stay in the hands of a Republican.”
Messrs. Perez and Sanders are in the midst of a coast-to-coast tour titled “Come Together and Fight Back,” but Mr. Sanders remains just as uninterested in coming together with the Democratic Party as he did while fighting its leaders during last year’s presidential primary campaign.
Mr. Perez, who plans to stump Thursday for Mr. Ossoff, sees electing Democrats as the key to recovery.
Mr. Sanders told a Maine crowd Monday night that “our job is to radically transform the Democratic Party.” He put the onus on Mr. Perez and other party leaders to adopt his aggressive power-to-the-people worldview.
“It can’t be just symbolic, it has got to be real,” Mr. Sanders said. “It has got to be that those ideas are allowed to become the dominant theme of the Democratic Party, and that’s the choice that Democrats are going to have to make.”
For Mr. Perez, a former Obama administration Labor secretary with little national profile, the symbolism of being seen stumping with Mr. Sanders is to highlight the party’s liberal wing and his embrace of it.
“When they see Tom Perez and Bernie Sanders in the same place talking about the same issues, that goes a long way toward their understanding that, you know what, if I want to see progress in this country, I need to align with the Democrats,” Mr. Perez said during an interview while riding between events in Frankfort and Louisville on Tuesday.
Mr. Sanders’s progressive test lies largely on economic issues, not social or cultural ones.
He plans to campaign Thursday in Nebraska with Heath Mello, a former Nebraska state senator who in 2009 sponsored legislation requiring women to look at ultrasound image of their fetus before receiving an abortion.
At the time Mr. Mello called the proposal a “positive first step” toward reducing the number of abortions in Nebraska. It became law months later.
Eight years later Mr. Mello remains opposed to abortion, said Nebraska Democratic Party Chairwoman Jane Kleeb, who said abortion hasn’t been an issue in Mr. Mello’s campaign for Omaha mayor.
“Voters know he’s pro-life but we have a lot of pro-life Democrats in our state,” Ms. Kleeb said. “It’s not the single issue people vote on anymore.”
Mr. Sanders called himself “100% pro-choice” and said that if Mr. Mello wins his May 9 election against an incumbent Republican it will energize Democrats to seek office in other conservative states.
“If this fellow wins in Nebraska, that would be a shot across the board, that in a state like Nebraska a progressive Democrat can win, that will give hope to folks in other conservative states that perhaps they can win as well,” Mr. Sanders said.
Mr. Perez said the Democratic Party platform supports abortion rights but doesn’t require its candidates to do so. “If you demand fealty on every single issue, then you know it’s a challenge,” he said.
In Kentucky, a state Mr. Sanders lost to Mrs. Clinton by less than 2,000 votes, the two wings of the Democratic Party have yet to come together, said Mary Nishimuta, a Sanders supporter during the primary who on Monday became the executive director of the Kentucky Democratic Party.
“I’m not going to sit here and sing ‘Kumbaya’ and say everybody is going to walk down a happy path together,” said Ms. Nishimuta, who hosted Messrs. Perez and Sanders Tuesday at a coffee shop she owns in Frankfort. She called the Perez-Sanders tour “a really important step to rebuild trust and rebuild the belief that the Democratic Party will fight for people.”
Even among party officials there a clear sense that last year’s elections damaged the Democratic brand among people who ought to make up the party base.
“There are a lot of people who have Democratic values who may not see themselves as a Democrat,” said Michael Blake, a New York state assemblyman who in February won a post as a DNC vice chairman.
Write to Reid J. Epstein at Reid.Epstein@wsj.com and Natalie Andrews at Natalie.Andrews@wsj.com
Corrections & Amplifications
Heath Mello is running for mayor of Omaha in an election May 9. An earlier version of this article incorrectly stated the election was May 14. (April 19, 2017)
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