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.
The results help determine the political landscape - and Mr Trump's standing within his own party - just three months before the mid-term elections in which the Republicans are defending their House and Senate majorities across the nation.
"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.
"Now Troy wins a great victory", Trump wrote. "He will win BIG in Nov".
The race also received extra national attention due to President Donald Trump's involvement, as he endorsed Balderson and stumped for the Republican candidate in OH last weekend.
The script for Ohio's special election was somewhat familiar: An experienced Trump loyalist, two-term state Sen. Pat Tiberi, was never even supposed to be in play.
The Democrat candidate Danny O'Connor had not conceded defeat by 11pm U.S. eastern time.
Republicans were hoping for Democratic discord in Kansas' 3rd Congressional District, a suburban Kansas City district where several candidates were fighting for the chance to take on Republican Rep. Kevin Yoder in November.
The Republican victory would mean Democrats will still need a net pickup of 23 seats to win the House this fall.
Image Troy Balderson held a slight lead but the election was ruled too close to call
On if one or the other has momentum: O'Connor comes into this with some momentum.
Trump took the district with 53 percent of the vote in 2016, just one-point below Mitt Romney's performance there in 2012.
"I've worked in OH presidential and senate races for Republicans and the idea of #oh12 being a close race is sort of like hearing gravity is a regional phenomena", tweeted Republican strategist Stuart Stevens.
On if the midterm elections are unlikely to be like 2010: I mean I'd be surprised if they lost half as many seats as they won in 2010, mostly because Republicans don't have as much exposure.
He also needed good numbers in neighboring Delaware County. In the all-important county of Shawnee, Kobach is down by about 100 voters to Colyer's 39 percent with only 1 out of 224 precincts reporting.
The night before the rally, Trump took to Twitter to attack LeBron James, calling the National Basketball Association star dumb just days after James, an OH native, poured tens of millions of his own dollars into the opening of an innovative public school in Akron.
Cory Bliss, executive director of the Congressional Leadership Fund, a Republican-aligned SuperPAC, said: "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".
The five-way Democratic primary featured labor lawyer Brent Welder, who campaigned recently with self-described democratic socialists Vermont Sen.
Meanwhile progressive duo Sen.
Michigan Democratic gubernatorial candidate Abdul El-Sayed campaigns with support from New York Democrat candidate for Congress Alexandria Ocasio-Cortez at a rally on the campus of Wayne State University on July 28, 2018 in Detroit, Michigan.