House Speaker Paul Ryan, left, is facing a challenge for his Wisconsin congressional seat from Democrat Randy Bryce. (Reuters/Facebook)

Randy Bryce, a union ironworker known as "Iron Stache," has won the Democratic primary in the race to replace retiring House Speaker Paul Ryan, overcoming a lengthy criminal history including a drunken-driving arrest and a multi-year child support delinquency.

Bryce entered the race months before Ryan's April retirement and raised about $6 million — six times what primary challenger Cathy Myers, a school teacher, brought in.

He also won endorsements from Vermont Sen. Bernie Sanders and labor activist Dolores Huerta.

Democrats are optimistic for their chances to pick up the southeast Wisconsin district that Ryan has represented for 20 years. However, opponents argue that Bryce's rap sheet makes him unreliable and unelectable.

Bryce has been arrested at least nine times, according to police records, including in 1998 for operating a vehicle while intoxicated. He pleaded guilty, but initially failed to appear in court; he ultimately received a suspended sentence.

In 1991, he was booked for marijuana possession, trespassing, and theft, among other charges. In 2000, he was arrested for driving with a suspended license; he was arrested for a similar charge in 2003.

In addition, the Milwaukee Journal Sentinel reported last year he was delinquent on his child support payments to his ex-wife for nearly two years. Bryce paid off $1,257 in child support debt just two months before he launched his campaign for Congress, according to the paper.

On the Republican side, Bryan Steil, a former driver for Ryan from a prominent family in his hometown of Janesville, won the GOP primary in the race to replace him Tuesday in the state's 1st Congressional District.

Bryan Steil, a former driver for Paul Ryan, won the GOP nomination to succeed him in Congress on Tuesday. (AP)

Ryan's retirement creates an opening in his southeastern Wisconsin congressional district for the first time in 20 years, fueling hopes among Democrats that they can pick up the seat that leans Republican.

But Steil is looking to keep the district bordering Illinois under GOP control, even if there's a Democratic "blue wave." Armed with Ryan's endorsement, Steil was the front-runner in a five-way Republican primary.

Ryan, who announced he would be retiring in April, has held the position of speaker since the retirement of former House Speaker John Boehner in 2015. His retirement will take effect in January.

The House speaker has had a rocky relationship with President Trump, and condemned Trump's more controversial moves on a number of occasions during the campaign. He faced pressure to step aside early as speaker in May, as the GOP-led Congress failed to pass a comprehensive immigration bill despite numerous attempts.

Steil is an attorney who has worked for a variety of manufacturers in the district. Since 2016, he's also served as a member of the University of Wisconsin board of trustees, appointed by Republican Gov. Scott Walker.

In other Wisconsin congressional races:

— 3rd District: In this Milwaukee district, seven-term Democratic incumbent Gwen Moore faced a longshot challenge from Gary George, a former state senator who was convicted of a felony in a kickback scheme in 2004 and ran unsuccessfully against Moore in 2014 and 2016. On the Republican side, deliveryman Tim Rogers faced Cindy Werner, a U.S. Army veteran who moved to Milwaukee 18 months ago from Texas.

— 5th District: In the suburban Milwaukee district, Republican Rep. Jim Sensenbrenner, the second-longest serving member of the House first elected in 1978, faced his first primary in a decade, with pediatrician Jennifer Vipond making her first run for office.

— 7th District: In this northern Wisconsin district, Democrats Margaret Engebretson, an attorney, and Brian Ewert, a doctor, were battling for a chance to take on Republican Rep. Sean Duffy.

Fox News' Kaitlyn Schallhorn and The Associated Press contributed to this report.