Two luminaries in the democratic socialist movement – one its national leader, the other its new star – are descending on solidly Republican Kansas on Friday, taking their emboldened liberal message to an unlikely testing ground before next month's congressional primaries.

Sen. Bernie Sanders (I) of Vermont and Alexandria Ocasio-Cortez, who rose to fame following her surprise win in last month's New York congressional primary, see an opportunity to influence Democratic voters in Kansas ahead of the state's Aug. 7 primary. They're especially focused on a crowded congressional primary in the Kansas suburbs of Kansas City.

In an election year defined by energized Democratic voters seeking to send President Trump a message, Senator Sanders and Ms. Ocasio-Cortez are betting they can stoke the liberal march in places where the left rarely competes. Some liberal voters are welcoming the spotlight.

"Progressive voters and even some moderate voters have been disheartened by the lack of positive news from Kansas," said Anne Black, a Democratic precinct committee member from suburban Kansas City, Kan.

The trip is unusual on several fronts. For one, Mr. Trump won Kansas in 2016 by 20 percentage points, making it seemingly inhospitable for Democrats, much less democratic socialists. Moreover, Sanders is a 76-year-old Jewish senator from Vermont, while Ocasio-Cortez is a 28-year-old Latina from the Bronx who is poised to become the youngest member of Congress.

This political odd couple is scheduled to headline an evening rally in Kansas City for Brent Welder, a labor lawyer running in a crowded Democratic primary in Kansas's 3rd District. The district, represented by four-term Republican Rep. Kevin Yoder, is on Democrats' target list as they aim to seize the GOP-controlled House in November. Democratic presidential nominee Hillary Clinton narrowly carried the district in 2016.

Sanders and Ocasio-Cortez are also scheduled to campaign together in Wichita, Kan., for Democrat James Thompson, a civil rights lawyer running in Kansas's 4th District. Like Ocasio-Cortez and Welder, Mr. Thompson was an activist for Sanders's 2016 presidential campaign.

While organizers were forced to change venues for the Wichita event because of high demand for the Friday afternoon rally, the race in the 3rd District is considered more competitive. Still, Republicans are skeptical.

State Rep. Tom Cox, a moderate Kansas City-area Republican, said there are pockets of liberal Democrats in the Kansas City suburbs but questioned whether Sanders's message will resonate more broadly. He said Democrats tend to be split between liberals and moderates, with some union members and supporters holding conservative views on social issues.

"Even our Democrats around here are not socialist democrats," he said. "If someone would describe the 3rd District, I would say center-right."

Democrats, who have been shut out of statewide and congressional races since 2010, are having a similar debate among themselves. They must pick up at least 23 Republican-held seats to claim the House majority, and they are focusing on 25 districts where Ms. Clinton won, or Trump won narrowly.

Leading candidates in the Democratic primary for governor have said their party must rebuild its brand in rural, heavily GOP areas. And despite surging energy among leftist Democrats in the Trump era, it was unclear if there were enough votes in the 3rd District for a liberal Democrat to win.

In 2016, Clinton narrowly won in this urban and suburban district whose neighborhoods are out of keeping with the agriculturally rich prairies that make up much of that state. And before Representative Yoder first won in 2010, it had been held for 12 years by centrist Democrat Dennis Moore, who relied on moderate Republicans during his tenure.

Yet Sanders and his brand of liberalism have proved popular. He won more than two-thirds of the votes in the state's 2016 presidential caucuses, surpassing President Barack Obama's 2008 vote total.

But registered Republicans in the 3rd District outnumber their Democratic counterparts by more than 50,000, while unaffiliated voters also edge Democrats. Republicans outnumber Democrats by 2-to-1 in the 4th District.

Liberals argue that they are not just convincing moderate Democrats or disaffected Republicans but also engaging new primary voters, as Ocasio-Cortez did in New York this summer and as Sanders did in his insurgent 2016 presidential campaign.

"If you're going to flip the district, you have to get new people involved in the political process," said Sanders spokesman Josh Miller-Lewis. "There are so many people not involved."

And candidates promoting Sanders's agenda have won Democratic primaries in several of these districts, such as in Orange County, Calif., and suburban Philadelphia.

A race for Kansas Democrats to watch lies just 180 miles north on Interstate 29.

Democrat Kara Eastman won the May primary against moderate former Rep. Brad Ashford on messages much like Sanders's in Nebraska's 2nd District, which includes the city of Omaha and its suburbs.

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She faces first-term Republican Rep. Don Bacon in this Republican-leaning district, where Trump won narrowly in 2016 but Mr. Obama won in 2008.

This story was reported by The Associated Press.