But the fact that this race was even competitive to begin with casts doubt on Republican prospects of staving off that outcome.
Trump took credit for the apparent Republican victory, saying on Twitter that Balderson's campaign made "a big turn for the better" after he campaigned for him in the district on Saturday night. He's running for a seat that has been held by a Republican since 1983, but his Democratic opponent, Danny O'Connor, is running surprisingly close to him. As the polls tightened in recent weeks, House Speaker Paul Ryan and Vice President Mike Pence appeared in the district, and national Republican groups poured millions into the race.
"When you look at the economy since he began, I'd say we're moving in the right direction and need more Republicans in there to support his agenda", he said. "Strong on Crime, Border & Military", the president tweeted on the eve of the election.
Should the polarizing Kobach win, some Republican operatives fear he could lose the governor's seat to Democrats this fall.
Laura Kelly will have a tough battle in the Republican stronghold of Kansas, but if she faces Mr Kobach she could play on his extremist profile to target centrist Republicans. The suburban-rural splits were huge, with O'Connor over-performing normal Democratic numbers near Columbus and Balderson racking up strong margins in the district's smaller towns and rural areas.
Just 20 minutes later, at 10:01 p.m. eastern, with 95 percent of precincts reporting, Balderson was back up for good by 741 votes.
Three mainstream Democrats are viewed as leaders to vie for retiring Republican Rep. Dave Trott's seat in November.
The pro-Balderson effort has focused largely on motivating Republican voters by casting O'Connor as extreme.
Mr 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. The Republican campaign arm and its allied super PAC were forced to pick up the slack, spending more than US$4 million (NZ$5.9m) between them. This was a particularly thorny issue in the home district of John Kasich, the popular Republican governor of the state who has remained an unabashed critic of Trump. The field included Fayrouz Saad, who would be the first Muslim woman in Congress.
Republicans are scrambling to avoid an embarrassing defeat in a seat that the party has held for decades. Two vulnerable Senate Democrats, Missouri's Claire McCaskill, and Michigan's Debbie Stabenow are facing challenges in their states. He would join Tim Scott of SC as the only black Republican senators if he wins in November. 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.