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With just days to go before the election, the three-way Republican Senate primary in Ohio has turned into a food fight, fueling concerns about former President Donald J. Trump’s favored candidate, Bernie Moreno.

The contest on Tuesday to decide who will face Senator Sherrod Brown has been contentious for months, with Mr. Moreno, a wealthy former car dealer who has never held elected office, struggling to outrun his rivals, State Senator Matt Dolan and Secretary of State Frank LaRose. But in recent weeks, a handful of independent surveys have indicated that Mr. Dolan, a more traditional conservative with deep pockets of his own, is gaining traction.

On Monday, Mr. Dolan received the endorsement of Gov. Mike DeWine of Ohio — after gaining backing last week from another statewide Republican, former Senator Rob Portman. That same day, Mr. Trump’s campaign announced that the former president would appear alongside Mr. Moreno on Saturday in Dayton, widely interpreted as a sign that Mr. Moreno could benefit from an 11th-hour boost. (The former president had planned to attend a rally in Arizona but was redirected because of concerns about Mr. Dolan's surge in internal polling, according to two people familiar with the planning.)

In the homestretch, Mr. Dolan and groups supporting him have outspent both Mr. Moreno and Mr. LaRose, blanketing the airwaves with attacks highlighting inconsistencies in Mr. Moreno’s record that could be of concern in a Republican primary, such as the more liberal views on immigration he espoused in the past. Simultaneously, Mr. Moreno and his backers have portrayed Mr. Dolan as not sufficiently supportive of Mr. Trump.

“This is between the steady-at-the-wheel, consistent conservatives over the last 20-plus years, versus the more upstart, populist, Donald Trump-inspired candidates,” said Ryan Stubenrauch, a Republican strategist in Ohio who has not endorsed any of the candidates.

He called Mr. DeWine and Mr. Portman “conservative, popular politicians who did a lot of good” in Ohio, adding, “That still counts for something, is what we’re seeing, and it’ll be interesting to see how much it counts for.”