Ohio Treasurer Josh Mandel abruptly ended his campaign for the U.S. Senate on Friday, dealing a blow to the GOP in one of the marquee contests of the 2018 elections.

In an email to supporters, the Republican cited an unspecified health issue his wife is battling as the reason for his decision.

"Understanding and dealing with this health issue is more important to me than any political campaign," Mandel wrote. "For as long as that takes, whether it is months or years, it is important that I heed my dad's advice and be there for my wife and our kids."

Mandel said he would continue to serve out the rest of his term as state treasurer.

The Republican launched his challenge against Sen. Sherrod Brown (D-Ohio) in late 2016. While he lost to Brown in 2012, Mandel was seen by GOP officials as a top recruit for the race, who had built name recognition in the state as well as a national fundraising network.

Brown's Senate seat is one of two-dozen that Democrats are defending this year, many in states that, like Ohio, President Trump won in 2016. Republicans, who hold a narrow 51-to-49 majority, are defending eight Senate seats.

The candidate filing deadline in Ohio is Feb. 7 — giving other Republicans only about a month to decide whether to enter the race.