WASHINGTON—House Speaker Paul Ryan, for the first time since entering Congress, may next year face a Democratic opponent who has a professionally managed campaign and the prospect of significant financial backing.

Randy Bryce, a local labor activist, is preparing a campaign launch in the coming weeks to challenge the Wisconsin House seat and has enlisted staff members including Bill Hyers, a Barack Obama alumnus who ran upset campaigns for New York City Mayor Bill de Blasio and Sen. Kirsten Gillibrand (D., N.Y.).

In an interview, Mr. Bryce said he would seek to tether Mr. Ryan to President Donald Trump and argue that their effort to dismantle large parts of the Affordable Care Act has hurt working people in the district.

“They’re handcuffed together,” Mr. Bryce said. “People are having buyer’s remorse and they’re seeing what’s going on. Trump made a lot of promises that I can see why working people would support, but now they’re waking up.”

Since he was first elected in 1998, Mr. Ryan has only once received less than 60% of the vote in his district, which covers parts of six counties in Wisconsin’s southeast corner. Mr. Ryan, who has built an image as an economic-policy specialist, is also one of the party’s most prolific fundraisers and has nearly $10 million in his campaign account. And only one House speaker since the Civil War has been turned out by his own constituents; Democratic Speaker Tom Foley lost his seat in the 1994 GOP wave election.