A win by Vance, who like Trump was a newcomer to politics and has been trailing in the polls, would demonstrate that the former president’s hold on his party’s base remains ironclad, likely discouraging Republicans who are considering challenging him should he run again.

But some Ohio Republicans warn that a Vance victory would endanger the party’s hold on the seat of retiring Republican Senator Rob Portman, making it more difficult for the party to win control of the U.S. Senate and block Democratic President Joe Biden’s legislative agenda.

More than 40 Republican officials wrote to Trump earlier this month urging him to not endorse Vance, arguing that his history of criticizing Trump would be fodder for his Democratic opponent and discourage Republican turnout in the November general election.