Rapinoe backs Warren for 2020 election

US soccer star Megan Rapinoe is backing Elizabeth Warren's bid for the US presidency [Courtesy]

United States soccer star Megan Rapinoe said Friday she is backing Democratic hopeful Elizabeth Warren in the 2020 US presidential election.

Rapinoe, one of the cornerstones of the USA's victory in the Women's World Cup in July, announced her endorsement for the progressive Massachusetts senator in a post on Twitter.

"I truly believe the best things in life are a result of being bold and being real," the 34-year-old said.

"I'm proud to endorse Elizabeth Warren today, for being bold, for being real, for listening to ALL of us, and for being prepared to navigate the unique challenges we face today as a country," Rapinoe added.

The post was accompanied by a video showing Rapinoe and Warren speaking in a phone conversation.

"I just wanted to call and say thank you, we need you in this battle," Warren tells Rapinoe in the conversation.

Rapinoe responds by saluting Warren's campaign for the presidency.

"I just think that it's amazing," Rapinoe says. "It's big, it's bold. I just don't think we can get to a better place by walking this moderate line."

Rapinoe has been a prominent supporter for progressive causes throughout her career, drawing attention this year for her advocacy of social justice off the field.

As well as leading the fight for equal pay for the US women's soccer team, she locked horns with President Donald Trump during the World Cup, when she reiterated that the team would not attend a White House reception if they won the tournament.

Warren is among the frontrunners in a crowded but narrowing field of Democrats who have thrown their hat into the race to be the nominee to challenge Trump for the White House in 2020.

She thanked Rapinoe for her endorsement in a separate tweet.

"I'm so grateful to have you on @TeamWarren. I promise to fight shoulder-to-shoulder alongside you for equal pay and the bold, structural change our country needs," she said.