A Monmouth poll last week showed the race was essentially a dead heat, down from a double-digit advantage for Balderson a month ago.
This is a special election, not a primary, so the victor of Ohio's vote tonight will head to Congress after votes are counted.
Voters in Kansas, Missouri, Michigan and Washington state joined those in OH in Tuesday's voting. But only OH will send someone to Congress immediately.
Democrat Danny O'Connor, the Franklin County recorder, speaks to the media alongside his fiancee Spenser Stafford, after Stafford voted at the Noor Islamic Cultural Center, Tuesday, Aug. 7, 2018, in Dublin, Ohio.
Trump took credit for the Republican candidate's performance, calling it "a great victory", even though the contest could be headed to a mandatory recount.
Trump again pushed Balderson in an early morning tweet before voting began.
A number of races on both sides have several candidates from which to choose, including five persons running for Kansas governor on the Democratic side and seven persons running on the Republican side. "I want to see someone who's interested in working with the other party so we can actually get something done".
But it isn't entirely clear if Trump's support will ultimately help or hurt Balderson on Tuesday. Debbie Stabenow. Trump endorsed James, who also had support from many in the GOP establishment in his race against self-funding venture capitalist Sandy Pensler.
Kansas Republicans were fighting among themselves in a battle for governor, where Secretary of State Kris Kobach was trying to unseat Gov. Jeff Colyer. He submitted petitions Monday with more than 10,000 signatures for what could become the most serious independent run for Kansas governor in decades.
O'Connor and his Democratic allies have railed against the tax plan, casting it as a giveaway for the rich that exacerbates federal deficits and threatens Medicare and Social Security. Senate Republicans had hoped to make MI, a state which Trump carried, competitive, but it's fallen down the list of top targets for 2018.
O'Connor has dominated Balderson on the local airwaves. "Strong on Crime, Border & Military", the president tweeted on the eve of the election.
After Alexandria Ocasio-Cortez's stunning victory in NY against Rep. Joe Crowley, progressives hope to demonstrate that their policies can win over voters in the middle of the country.
Senate contests in Missouri and MI were coming into focus as two vulnerable Democrats, Missouri's Claire McCaskill and Michigan's Debbie Stabenow, were expected to easily claim their party's nominations.