A Monmouth poll last week showed the race was essentially a dead heat, down from a double-digit advantage for Balderson a month ago.
Doug Jones (D-AL) as well as numerous other wins in legislative special elections and moral victories in other congressional special elections.
Voters in OH and Kansas joined those in Missouri, Michigan and Washington state.
Polls have closed in Ohio's 12th Congressional District, Missouri, Michigan and Kansas.More news: Ollie Pope plans to replicate Curran´s class in Test arena
The Cook Political Report, which monitors congressional races across the country, lists the Lowcountry 1st District, long held by Republicans, as a potential pickup for Democrats this fall.
The Republican president's shadow also loomed over primary contests in four other states, none bigger than Kansas, where Trump roiled the governor's race by opposing the GOP incumbent on the eve of the election. Republicans had tried to tie the 31-year-old Franklin County recorder to House Minority Leader Nancy Pelosi, falling back on a tried and true strategy of scaring wayward Republicans and independents about a return of Speaker Pelosi.
Democrats must pick up 23 seats in the House and two seats in the U.S. Senate in November to gain control of those chambers and put a brake on Trump's agenda.
Trump again pushed Balderson in an early morning tweet before voting began. It's an enormous amount of exposure for their party and we don't see that for the Republican party.
But it isn't entirely clear if Trump's support will ultimately help or hurt Balderson on Tuesday.More news: New Jersey woman spots Keith Urban cash for snacks at Wawa
But the GOP candidate in the race, Troy Balderson, is a Trump supporter who recently welcomed President Trump at a town rally this weekend.
Meanwhile, more than 700 miles to the west, Kansas Republicans were fighting among themselves in the battle for governor, where Secretary of State Kris Kobach was trying to unseat Gov. Jeff Colyer.
O'Connor and his Democratic allies railed against the tax plan, casting it as a giveaway for the rich that exacerbates federal deficits and threatens Medicare and Social Security.
The pro-Balderson effort has focused largely on motivating Republican voters by casting O'Connor as extreme. Strong on Crime, Border & Military.
In Kansas' 3rd District, Bernie Sanders-backed labor lawyer Brent Welder is running in a six-way primary to take on Yoder. Bernie Sanders and ascending political star, Alexandria Ocasio-Cortez, a NY congressional candidate.More news: Mendocino Complex now California's largest wildfire in history
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.