The Republican congresswoman Liz Cheney has now endorsed two Democrats for election in the midterms next week by backing Tim Ryan in his Ohio US Senate race against JD Vance after endorsing Elissa Slotkin for re-election to the US House in Michigan.

In Cleveland, Ohio, on Tuesday, Cheney told the journalist Judy Woodruff she backed Ryan, currently a Democratic congressman, over Vance, the Hillbilly Elegy author and venture capitalist who abandoned criticism of Donald Trump in order to publicly embrace him.

“I would not vote for JD Vance,” Cheney said.

Asked if she would vote for Ryan if she lived in Ohio, she said: “I would.”

In an unexpectedly tight race, the polling website FiveThirtyEight puts Vance two points up.

The daughter of the former vice-president Dick Cheney is a stringent conservative but has nonetheless emerged as perhaps the leading anti-Trump Republican. She rose to national prominence as vice-chair of the House January 6 committee, seeking to establish Trump’s guilt regarding the US Capitol attack, but in August lost her primary in Wyoming to a Trump-backed challenger.

Speculation continues over whether Cheney will mount a presidential run in 2024, whether for the nomination in a party dominated by Trump or as an independent. She has not confirmed or denied any plans.

She has said she will leave the Republican party if Trump is the nominee again.

Cheney endorsed Slotkin last week, saying: “While Elissa and I have our policy disagreements, at a time when our nation is facing threats at home and abroad, we need serious, responsible, substantive members like Elissa in Congress.”

It was Cheney’s first ever endorsement of a Democrat. She was due to campaign for Slotkin on Tuesday evening.

Earlier on Tuesday, Slotkin told CNN: “While we disagree – and I look forward to debating her in the future on issues of policy – on democracy, we are in vehement agreement. We agree on one really big thing and that’s that there has to be a democratic system … It’s pretty important to me.”

The endorsement happened, Slotkin said, after the Republican approached the Democrat on the House floor in September.

“She just said, ‘Look, if there’s anything I can do to help, you just let me know,’ and I said, ‘Really?’ and she said, ‘Yeah,’ and that began the conversation. So I’m appreciative because it’s again just another point of bravery that I think I really admire about her.”

Before entering politics, Slotkin was a CIA analyst under Barack Obama, a Democrat, and George W Bush, the Republican to whom Dick Cheney was vice-president.

Slotkin is running in a newly drawn district against a Republican state senator, Tom Barrett, in what has by some measures become the most expensive midterms race.

With a week to go until polling day, Slotkin is pinning her hopes on opposition to Republican abortion bans and the supreme court ruling which this summer overturned the constitutional right to abortion. Cheney has said she remains an opponent of abortion rights.

On Tuesday, the polling website FiveThirtyEight gave Slotkin a 74% chance of victory.