Speaking at a rally two days ahead of voting in a heated Republican Senate primary in Ohio, Donald Trump appeared to forget the name of JD Vance, the candidate he has endorsed.

“You know,” the former president told a crowd in Greenwood, Nebraska, on Sunday, “we’ve endorsed Dr Oz.”

So he has, in Pennsylvania.

Of Ohio, Trump said: “We’ve endorsed – JP, right? JD Mandel, and he’s doing great. They’re all doing good. They’re all doing good. And let’s see what happens.””

Trump appeared to be confusing JD Vance, a former US marine, author of the bestseller Hillbilly Elegy and venture capitalist, with Josh Mandel, a rival who courted Trump for the endorsement.

Trump also said: “I think Vance is doing well, I think Oz is doing well. Does everybody like Oz? I love Oz.”

Announcing his endorsement last month, Trump said: “In the great state of Ohio, the candidate most qualified and ready to win in November is JD Vance. We cannot play games. It is all about winning!”

Vance said then he was “incredibly honored to have President Trump’s support”. He did not immediately comment about Trump’s slip on Sunday.

Trump’s endorsement – despite past remarks in which Vance called Trump “America’s Hitler” and “a moral disaster” – boosted Vance to the top of the polls in Ohio. On Sunday, Mandel continued to hammer his opponent for such remarks, including saying in 2016 he might vote for Hillary Clinton if it seemed Trump might win.

Another candidate, Mike Gibbons, told donors in an email reported by Politico: “To be fair, you really can’t blame Trump. No one knows who the real JD Vance is, as his views change faster than the weather in Ohio.”

In the same rally, Trump, 75, mocked Joe Biden, 79 and the president who beat him in 2020, for stumbling over his words. Biden has often discussed his struggle to overcome a stutter.

Amid widespread delight over Trump’s Vance-Mandel slip, the columnist Julia Davis referred to a notorious interview in which Trump discussed passing a cognitive test which he insisted was “difficult”.

Davis wrote: “Person. Woman. Man. Camera. TV. JP. JD. Mandel. Oh hell.”

Trump’s rally on Sunday was in support of Charles Herbster, a candidate for governor in Nebraska who – like Trump – faces accusations of sexual assault and misconduct.

“I defend people when I know they’re good,” Trump said. “A lot of people, they look at you and say: ‘You don’t have to do it, sir.’ I defend my friends.”