Hillary Clinton not only expected a possible challenge from Sen. Elizabeth Warren, D-Mass., but also considered her as a potential running mate, according to her new book What Happened.

Clinton says Warren's name was "most often mentioned," as one of her strongest challengers from the Left but over time she came to view her as less of an adversary and more of a champion.

She recounts how the two met at her home in Washington's Georgetown neighborhood, admitting the two were initially "a little wary" of each other.

"I came away convinced that if Elizabeth believed her views and priorities would be included and respected in my campaign, she might become my champion rather than my challenger," Clinton wrote.

The failed Democratic presidential candidate went on to name Sen. Time Kaine, D-Va., as her vice presidential pick but acknowledges the Massachusetts senator was vetted a possible running mate.

"Elizabeth was on my list of potential choices for Vice President," Clinton said.

Warren has been floated as a potential 2020 candidate but continues to say it is too early to speculate about the next presidential race and that she is focused on being a champion of liberal causes.

"I'm serious. Donald Trump has been president for seven months. I mean, I know it seems like seven years, but it's been seven months," Warren recently told WGBH.

"We can't be a party and we can't be a nation that says, ‘Horse race to horse race, let's just see what happens every four years and let's get all excited about speculating who's going to be in the race in four years," she added.