Lucia McBath, second from left, and fellow members of Mothers of the Movement address the Democratic National Convention in Philadelphia on July 26, 2016. (Ricky Carioti/The Washington Post)

Democratic voters in suburban Atlanta will choose nominees Tuesday in two congressional districts where their party has the best shot of picking off seats that have been reliably red.

Although political handicappers give Republicans the advantage, the races in the 6th and 7th districts could be competitive in November because of changing demographics and because President Trump underperformed in them in 2016.

But first Democratic voters must decide which candidates would be best to take on the GOP incumbents.

Lucia “Lucy” McBath, whose teenage son’s murder several years ago pushed her into the national debate on gun safety, and Kevin Abel, who owns a technology consulting firm, are competing in the 6th District. The winner will face freshman Rep. Karen Handel.

In the neighboring 7th District, another Democratic runoff is playing out between Carolyn Bourdeaux, a college professor, and David Kim, who founded a national company that provides tutoring for students taking classes and standardized tests. The winner of that contest will compete against Republican Rep. Rob Woodall.

McBath, a former airline flight attendant, was the top vote-getter in the May 22 primary, with more than 36 percent of the vote in the four-way race. She began the election cycle as a candidate for the Georgia legislature but said the February school shooting in Parkland, Fla., persuaded her to instead run for Congress to continue her advocacy for gun safety. Her only child, Jordan Davis, 17, was fatally shot in 2012 by a 47-year-old man who argued with the teenager and his friends that the music in their car was too loud.

McBath’s campaign has attracted national attention and support. Everytown for Gun Safety has spent more than $1 million to help McBath, who is on leave from her post with the group. She also has been endorsed by Emily’s List, Sen. Kamala D. Harris (D-Calif.) and Hillary Clinton, for whom McBath campaigned in 2016.

Abel, the runner-up, got 30.5 percent of the vote. He has sought to cast himself as the local candidate, criticizing McBath as being too beholden to national interests. His campaign manager also described McBath as a single-issue candidate. Charlie Blaettler said Abel “is in 100 percent agreement with Ms. McBath about what we need to do in this country to enact gun safety legislation,” but he is also talking about health care, immigration and the economy.

Rebecca Walldorff, a spokesman for McBath’s campaign, said that although “the animating event in Lucy’s decision to enter public life was the tragedy of her son’s murder,” McBath also lists fighting for health care and education as issues important to her candidacy.

McBath has questioned Abel’s allegiance to the Democratic Party, calling him out for a blog post he wrote in 2016 in which he discussed the idea of an independent third party that would, depending on the issue, work with Democrats and Republicans to forge compromise. Abel has been described in local newspapers as a moderate who can appeal to Republican and independent voters, but Blaettler declined to say whether Abel considers himself a moderate or progressive.

The 6th District has elected Republicans since 1979, when Newt Gingrich won the district and went on to help to lead the 1994 Republican takeover of Congress. The seat became vacant early last year when Trump tapped longtime Republican Rep. Tom Price to serve as his secretary of health and human services.

Democratic candidate Jon Ossoff, then a 30-year-old documentary filmmaker and former congressional aide, raised more than $20 million and became a symbol of the national resistance movement to push back against Trump and the Republican Congress’s conservative agenda. But Ossoff  fell two percentage points shy of topping the 50 percent of the vote in the first round and was forced into a runoff with Handel. In the second round Handel, a former secretary of state, won by four percentage points and became the state’s first Republican congresswoman.

Ossoff’s strong showing last year has given some Democrats hope that the seat is still within reach. Trump won the 6th District by 1.5 points, and Handel could be pushed aside if a blue wave hits Georgia this fall. Others believe that the 7th is a more likely pickup, because about 54 percent of its residents are people of color and Gwinnett County, which makes up a most of the district, went for Clinton in 2016. Yet that race has not attracted as much attention.

Bourdeaux, a public policy professor at Georgia State University, came in first place in the May with 27 percent of the vote among six candidates. Kim was a close second, with 26 percent of the vote.

Both candidates have vowed to fight to protect the Affordable Care Act and have been critical of Trump’s rhetoric and policies.

Kim has touted his success in business; his national firm, C2 Education, grew out of a tutoring service he launched while he was a student at Harvard University. He has criticized Bourdeaux, a former director of the state Senate’s budget office, for overseeing cuts to some education programs during the economic downturn.

Bourdeaux has embraced her work in government — she also was a former congressional aide — as evidence of her commitment to civic service. She has criticized her opponent for not having voted in 2016 — indeed, he had not voted until he decided to run for office this year.

Kim, whose parents are Korean immigrants, said voting was not part of his family’s culture. But he said Trump’s election was a wake-up call. In an interview with a local television station he said he was “very apologetic about not having been part of the fight in 2016.”

Charles Bullock, a political-science professor at the University of Georgia, said women make up a larger share of the Democratic electorate, a factor that could help McBath and Bourdeaux in their runoff contests. In the general election, political party affiliation will likely be a more important factor in how voters behave.