Former president George W. Bush is holding a fundraiser next month with Rep. Liz Cheney, as the Wyoming Republican and prominent critic of former president Donald Trump faces a tough reelection battle.

The event marks a rare campaign-trail appearance by Bush. Cheney is the daughter of Richard B. Cheney, Bush’s former vice president.

Trump has vowed to unseat Cheney since she voted to impeach him in January over his role in the Jan. 6 attack on the U.S. Capitol, which was carried out by a mob of supporters who echoed his false claims about the election while seeking to stop Congress from certifying its results and declaring Joe Biden the president-elect.

Earlier this month, Trump endorsed Cheney’s primary opponent, Harriet Hageman. The contest will serve as the marquee test of the former president’s ability to purge his critics from the party.

News of the fundraiser was first reported by Politico. A person familiar with the matter confirmed the event to The Washington Post.

According to an invitation for the fundraiser, several other top Republicans in Bush’s orbit are expected to attend the Oct. 18 event in Dallas. They include former Bush political adviser Karl Rove, former Bush senior adviser Karen Hughes, former White House counsel Harriet Miers, and former senator and U.S. Ambassador to NATO Kay Bailey Hutchison.