Bernie Sanders has stepped in to defend a progressive political agenda after Joe Biden suggested Democrats such as Alexandria Ocasio-Cortez could not defeat Republicans in a competitive race.

In an interview with CNN, Mr Biden said his centre-left approach was more in tune with most of his party, than that of progressives such as the New York congresswoman. He said Ms Ocasio-Cortez was “brilliant” and “bright” but that she had been elected after winning a primary in a Democratic stronghold.

“That’s what this election is about. I’m happy to debate that issue and all those issues with my friends because guess what, look who won the races. Look who won last time out,” said Mr Biden, 76, who remains the party’s 2020 frontrunner but who has seen his lead slip amid questions about his age and whether his policies are out of touch.

“By the way, I think Ocasio-Cortez is a brilliant, bright woman, but she won a primary. In the general election fights, who won? Mainstream Democrats who are very progressive on social issues and very strong on education and healthcare.”

Mr Sanders, on whose presidential campaign Ms Ocasio-Cortez worked in 2016, was quick to defend his political soulmate, who is almost 50 years younger than him.

Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures Show all 15 1 /15 Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures Alexandria Ocasio-Cortez greets fellow lawmakers ahead of the State of the Union address Getty Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures Alexandria Ocasio-Cortez participates in an event with Democratic members of Congress EPA Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures The Democrat senator speaks during a news conference at the U.S. Capitol January 30, 2019 Getty Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures Alexandria Ocasio-Cortez and fellow Democrat Rashida Tlaib AP Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures Alexandria Ocasio-Cortez at the Senate chamber to watch two votes on January 24, 2019 Getty Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures Alexandria Ocasio-Cortez arrives with Chellie Pingree at a House Democratic Caucus meeting Getty Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures Ocasio-Cortez during Donald Trump's State of the Union address Reuters Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures New York State Assembly member Catalina Cruz with Ocasio-Cortez AFP/Getty Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures Nydia Velazquez talks with Alexandria Ocasio-Cortez Reuters Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures Ocasio-Cortez casts her vote for Nancy Pelosi as Speaker of the House EPA Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures Alexandria Ocasio-Cortez poses with a campaign worker during a whistle stop in the Queens borough of New York Reuters Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures Ocasio-Cortez outside the US Capitol AFP/Getty Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures Ocasio-Cortez after casting her ballot in the 2018 midterm general election at a polling site in New York EPA Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures Alexandria Ocasio-Cortez looks on during a march organised by the Women's March Alliance in Manhattan Reuters Alexandria Ocasio-Cortez - the Democratic congresswoman in pictures Ocasio Cortez looks on at the Capitol in Washington, DC, on January 16, 2019 AFP/Getty

“I’m proud to be working with @AOC and so many other Democrats to pass Medicare for All, debt free college and a Green New Deal,” said the 77-year-old Vermont senator.

“This is the agenda America needs — and that will energise voters to defeat Donald Trump.”

The spat highlights a fierce debate taking place among Democrats, that somewhat echoes that of 2016, when Mr Sanders led a tough primary challenge to Hillary Clinton, a centrist.

One way in which the situation is different in this election is that a number of proposals Mr Sanders and Ms Ocasio-Cortez have been pushing, such as Medicare for All, have been adopted by many of those seeking the 2020 nomination, including figures such as Elizabeth Warren.

Alexandria Ocasio-Cortez speaks out against 'unconscionable' living conditions in migrant detention camps

While respected by many in the party, Mr Biden has been under intense scrutiny as Democrats work to select a challenger to Donald Trump. While the former vice president has sought to suggest he is best equipped to defeat the president in 2020, others have questioned whether the ideas he represents are the best strategy.

Mr Biden is almost certainly correct in is comments that his place on the political spectrum is closer to that of more Americans, than Ms Ocasio-Cortez, and he may be closer to more Democrats than she is – something the congresswoman would likely dispute.

“Look, it’s centre left, that’s where I am,” Mr Biden said of the party. “Where it’s not, is way left.”