Cheney and her father Dick Cheney, former vice-president under George Bush, are two of the most prominent Republicans to have endorsed Harris against her opponent Donald Trump.
Both have sharply criticised Trump, the Republican nominee, calling his refusal to accept the 2020 election results and role in the Jan 6, 2021 attack on the US Capitol disqualifying.
To win what polls show to be a very tight race with Trump, Harris needs to win over Republicans and independents wary of him, without alienating her base, particularly in states like Wisconsin that are likely to decide the election.
She has adopted a centre-to-right-leaning stance on several issues, including her staunch support for Israel, a border policy tough on migrants, and an all-of-the-above energy strategy to keep fuel costs low.
However, recent polling shows Harris is struggling to gain traction with Republican voters despite hundreds of former and current Republican officials in the military, national security and local governments having publicly backed her.
While Harris led Trump 47% to 40% among all voters in a Sept 20-23 Reuters/Ipsos poll, only 5% of the poll’s Republican respondents said they would back her in November’s election.
Around 10% said they would vote for another candidate, did not know who they would vote for or would not vote.
At Thursday’s event, Harris is expected to praise Cheney – a staunch conservative who opposed gay marriage and praised the Supreme Court’s repeal of the right to abortion – for putting her country above her party.
Harris plans to repeat her message from the Democratic Convention that she would be a president for all Americans regardless of party, and she will describe Trump as unfit to hold the office.
The event is set to take place in Ripon, Wisconsin at a one-room schoolhouse significant to the Republican party: it was the site of meetings that led to the party’s formation in 1854 and is referred to as the party’s birthplace.
Mary Anna Mancuso, a Republican strategist, said Harris was running “one of the most conservative platforms we’ve seen recently,” but said she thought there was little risk that a large number of liberals would reject Harris because of it.
“Where else would they go? They’re not going to go and vote for Donald Trump,” she said.