Former Rep. Liz. Cheney, R-Wyo., said she spoke with Vice President Kamala Harris in the process of making her endorsement, choosing to back her this week despite their party and policy differences.

Cheney told "This Week" co-anchor Jonathan Karl she would not get into the details of the conversation she had with Harris but praised her recognition of the stakes of the race against former President Donald Trump.

"I would just say that that she has a full recognition and understanding of the stakes of this race and of the importance of reflecting the broad coalition that's coming together to support her and voting for her to stop Trump," Cheney told Karl.

Harris and Trump are set to face off in their only scheduled debate of the 2024 election on Tuesday, moderated by ABC News. The debate will air at 9 p.m. ET on ABC and stream on ABC News Live, Disney+ and Hulu.

The endorsement from Cheney, made official this week, marks a stark turnaround for someone long considered to have a strong conservative record in the GOP. Cheney also announced last week that her father, former Vice President Dick Cheney, would also endorse Harris after years as a top Democratic bogeyman.

Cheney, a long known for her hawkish foreign policy views, told Karl that while she may disagree with Harris on policy, including foreign policy, Harris' fundamental commitments to international alliances like NATO stands in contrast to Trump's skepticism of America's obligations abroad.

"You can watch in terms of the lessons that that she learned as vice president. Listen to her talk about her vision for this country, her vision for the future, Cheney said when asked what kind of president Harris would be. "When it comes to fundamental alliances, when it comes to the importance of NATO, for example, and how important it is for the United States to lead in the world, we've seen a sea change. We now have a Republican Party that is embracing isolationism."

"And I believe strongly that if you're talking about a national security set of issues and you care about America's leadership role in the world, a vote for Vice President Harris is the right vote to make this time around."

When pressed on who she thought former President Ronald Reagan -- a Republican icon -- would vote for this year, Cheney was adamant that it wouldn't be Trump.

"There is absolutely no chance that Ronald Reagan would be supporting Donald Trump. Donald Trump doesn't stand for any of the things that Ronald Reagan did, and it's another place where I would urge my Republican colleagues, both in the Congress but across the country, to really look at Donald Trump's policies, to really look at the danger that he presents, to look at what he was willing to do to stay in power," Cheney said.

Cheney said Trump represents "a firm rejection, not just of traditional Republican policies, but of the constitutional order on which this country depends."