President Trump campaigned for Balderson, a 56-year-old state senator, this past weekend in an effort to energize the GOP base, but some Republicans anxious that the president's appearance could have had an adverse effect by motivating voters who don't like Trump to come out for O'Connor.
Voters on Tuesday were choosing between Balderson, a state senator, and O'Connor, the Franklin County recorder, to complete the term of a Republican who retired in January. A Republican advocacy group aired an ad featuring praise for Balderson, a state lawmaker, from Ohio Governor John Kasich, a frequent critic of Trump.
The Ohio race - the last special congressional election before November - is one of two taking place on Tuesday into which President Donald Trump has injected himself, turning both into new tests of Trump's tactics and sway with Republican voters.
O'Connor and Balderson exchanged leads as the votes rolled in, with the Democrat ahead by about one percentage point, 50 percent to 49 percent, with about 75 percent of precincts counted in the fight for a U.S. House of Representative seat.
Ultimately, the results may not even be known for a while. In the 2016 GOP presidential primary, though, Trump lost the state to conservative Sen.More news: Italian police: 2 dead, 60-70 injured in explosion
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The two races will garner the most national attention on a day when four states hold primary elections: Kansas, Michigan, Missouri and Washington. Outgoing Republican Gov. Rick Snyder had backed Calley, but Trump is more popular there with the GOP base in the state.
The Democratic race for governor in MI offers the next test of the electoral power of the party's progressive wing, with former Detroit health director Abdul El-Sayed vying to become the country's first Muslim governor against a more moderate Democrat, former state Senate leader Gretchen Whitmer.
Whitmer defeated Abdul El-Sayed, a former Detroit health commissioner.
On the left, candidates backed by Democrat-socialist darlings Alexandria Ocasio-Cortez and Bernie Sanders face off in key primaries in MI and Kansas.
But Democrats believe momentum from this spring's protests over education funding against Republicans who control the state government could make the state surprisingly competitive this fall.