The results would help determine the political landscape - and Trump's standing within his own party - as the GOP defends its House and Senate majorities this fall.
But Democrats - buoyed by enthusiasm ever since the 2016 presidential election - have consistently overperformed in other special elections, and notched upset victories in a Pennsylvania House race in March and in the Alabama Senate race previous year.
The day's races, which also included contests in Kansas, Missouri, Michigan and Washington state, pitted Mr Trump's fiery supporters against the Democratic Party's anti-Trump resistance. It was also a sign that the party's strategy for the race - fully embracing President Donald Trump and his bombastic message in hopes of motivating Republican voters, rather than trying to soothe moderates' worries about the President - could backfire in similar districts in November's midterm elections.
In Michigan, three mainstream Democrats in suburban Detroit were among those vying for a chance at retiring Republican Rep. Dave Trott's seat in November.
"While we won tonight, this remains a very tough political environment and moving forward, we can not expect to win tough races when our candidate is being outraised", Corry Bliss, the head of the big-spending Congressional Leadership Fund, warned in a Tuesday night statement.
"We're not stopping now", O'Connor said.
Polls are now open for today's Kansas Democratic and Republican primaries.
O'Connor has positioned himself as a moderate Democrat, pledging that if elected he would not vote for Nancy Pelosi as speaker of the House.More news: Trump claims Americans need ID to go shopping
In a way, Trump's popularity in the area is on the ballot, too.
Mr. Trump's fingerprints were on several of the contests, including in Kansas where he threw his support behind Secretary of State Kris Kobach, vice chairman of the White House's now defunct voter commission, in his race against Gov. Jeff Colyer, who took over after Gov. Sam Brownback resigned to accept an ambassadorship.
But the fact that this race was even competitive to begin with casts doubt on Republican prospects of staving off that outcome.
Voters in OH and Kansas joined those in Missouri, Michigan and Washington state. His pick for governor, Bill Schuette, the state's attorney general, easily won the GOP primary.
Four minutes later, at 11:01 pm eastern, President Trump declared victory.
Although the razor-thin race had not been called by any major news organizations, and was automatically going to a recount, Balderson, 56, delivered a victory speech and both the National Republican Congressional Committee and Trump put out statements declaring victory for their preferred candidate.
In the MI governor's race, progressive favorite Abdul El-Sayed, the 33-year-old former Detroit health official vying to become the nation's first Muslim governor, faces former state Senate Democratic leader Gretchen Whitmer, who is considered the favorite in the race. El-Sayed had the backing of top national progressives, including Vermont Independent Sen.
Balderson also got a boost from second-term Ohio Gov. John Kasich, a Republican who endorsed him late in the race, even as he sounded an alarm over the Trump-driven tactics, warning that they had alienated suburban women, in particular.