Ohio Treasurer Josh Mandel was once widely considered a rising star in the GOP — an earnest, baby-faced veteran and young father. He now is on his way to branding himself as the party’s untouchable for his tastelessness and opportunism.

It’s getting painful to watch a candidate for next year’s U.S. Senate race sink to such depths.

In 2010, his campaign for state treasurer engaged in anti-Muslim bigotry. And has continued to do so.

In July, Mandel — a Jew — slammed the Anti-Defamation League after the Jewish civil-rights group produced a report on white-supremacist hate groups. Mandel called the ADL “a partisan witch hunt group” and took to Twitter to “stand with” —or rather, slither with — a man who has claimed that date rape doesn’t exist and that “diversity is a code for white genocide.”

Only two months earlier, Mandel got smacked down by his own party leadership at the Statehouse, having spent nearly $2 million on self-promotional television ads starring himself and Ohio State football coach Urban Meyer.

The commercials came as Mandel announced his 2018 Senate rematch against incumbent Democrat Sherrod Brown. And the taxpayer-funded TV ads were paid for in multiple increments of less than $50,000 — a limit that triggers approval by a state oversight panel. It was like buying a billboard at a time.

Out of embarrassment or rectitude, the GOP slipped the “Mandel amendment” into the state budget to prevent such mischief in the future by requiring state Controlling Board approval of such expenditures.

Questions about Mandel’s profile-raising ads grew louder this summer when Cleveland.com reported that Mandel’s own marketing plan had said big-budget TV commercials were not the best way to promote a savings program targeted at the disabled.

In August, curious about what Mandel and his top aides were thinking when they decided to buy so much air time and pay for it piecemeal, The Dispatch requested all written communications on this between Mandel and five top staffers.

Most government offices don’t order the staff lunch without a flurry of emails. But last week, Mandel’s office responded by maintaining that it had sent zero emails or texts discussing the strategy of the ad buys.

Cleveland attorney and open-records expert David R. Marburger told us: “It’s entirely not plausible that a state agency would find a way to spend $2 million on advertising without internal discussion about it or written communication about it.”

Catherine Turcer, executive director of Common Cause Ohio, said: “My educated guess is they, in fact, deliberately avoided putting anything in writing.”

No paper trail exists because Mandel knew what he was doing was wrong. Or maybe he was just exceedingly reckless in spending $2 million in taxpayer funds without the usual written communications that debate and document such decisions.

On Tuesday, Mandel lashed out at reporters asking about his TV-ad caper, saying, “Shame on liberals in the media like yourself. Shame on radicals like Sherrod Brown for politicizing a program that is completely not political.”

Mandel then bolted, refusing to answer further questions — clearing up for the public exactly where the shame lies.