With all precincts reporting, Balderson, an OH state senator who closely linked himself to President Trump - appearing with him at a rally last weekend - held a 0.9 percent lead over O'Connor, a county recorder and moderate Democrat.
Troy Balderson, is fighting off a strong challenge from a fresh-faced Democrat, 31-year-old county official Danny O'Connor, in a congressional district held by the Republican Party for more than three decades. Check back for updates throughout the evening as votes are counted.
However, Tiberi resigned before finishing his term, forcing the special election to replace him.
But Democratic nominee Danny O'Connor has yet to concede - and the Associated Press has yet to call the race.
But Trevor Moffitt, a public health doctoral student at The Ohio State University who voted for O'Connor, said he felt Balderson's attacks on Democrats went too far.
The script for Ohio's special election is perhaps familiar: An experienced Trump loyalist, two-term state Sen.
The Ohio contest drew an avalanche of national attention in recent weeks as polls showed it tightening, with Republicans and their allies outspending Democratic groups by more than 4 to 1 and running ads linking O'Connor to liberal House Democratic leader Nancy Pelosi. The Democratic candidate, meanwhile, vowed: "We're not stopping now".
Ohio Republicans argued that the race should be taken as a warning, but not a reason to panic.
The results of the race for Ohio's 12th congressional district, which was spun by some experts and pundits as a test for the possibility of a "blue wave" that could hit the Midwest this November, actually led to another Republican representing the northern Columbus suburbs, Delaware County, and the nearby foothills region until the beginning of 2019. Now, whether the Trump rally over the weekend blunted some of that momentum we don't have the polling to see right now.
Why? With 100 percent of precincts reporting, and with the early vote counted, Balderson leads O'Connor by 1,754 votes out of 202,521 votes cast.
Kansas, Michigan, Missouri and Washington also held their regularly scheduled primaries on Tuesday, and the president endorsed candidates in several GOP races across those states. President Trump came out against the sitting GOP governor on the eve of that state's vote. "Now Troy wins a great victory during a very tough time of the year for voting", Mr Trump tweeted before the result had been announced. Outgoing Republican Gov. Rick Snyder had backed Calley, but Trump is more popular there with the GOP base in the state.
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. Bernie Sanders, Vermont independent, and Alexandria Ocasio-Cortez, the newest socialist star, made several endorsements in Tuesday's primaries.
The primary also is when the next congressman or congresswoman will effectively be chosen in two solidly Democratic U.S. House seats opening due to retirements, and when nominees will be selected for an open battleground district in suburban Detroit.
On stage with Trump, Balderson called himself "someone who will fight for President Trump's economic agenda".