US presidential candidate Bernie Sanders says it would be a "terrible idea" for the Democratic Party to recruit Vice President Joe Biden to run for president if Hillary Clinton withdrew from the presidential race due to her email controversy.

"I think that would be a terrible, terrible idea," Sanders said on Friday in an interview with The Young Turks, a popular online news show in the US.

"That would say to the millions of people who have supported us, that have worked with us, that would say all of your energy, all of your votes, all of your beliefs are irrelevant. We're going to bring in someone else," said Sanders, an Independent senator from Vermont who is seeking the Democratic Party’s nomination.

"I happen to like Joe a lot, but I think that would be a very, very serious blunder for the Democratic Party," he added.

Former US House Speaker John Boehner has said that it is very likely that Clinton will withdraw from the presidential race due to her email scandal and be replaced by Biden.

Boehner, who resigned from the US Congress last year, said on May 12 he "would not be surprised at all" if Clinton "has to withdraw" and Biden "parachutes in."

Republicans have long speculated that the federal investigation into Clinton's use of a private email server while she was US secretary of state could result in criminal charges against her, forcing Biden to seek the nomination.

Biden, however, has said repeatedly he is not interested in running for the White House, citing the death of his son Beau last year as the main reason.

Clinton is facing a potential indictment as the Federal Bureau of Investigation (FBI) investigates the email controversy.

More than 2,000 emails sent and received by Clinton while working as the top US diplomat between 2009 and 2013 include classified information, which the government bans from being handled outside secure, government-controlled channels.

Clinton is maintaining a massive lead over Sanders in the Democratic presidential race, but the senator has vowed to stay in the race until the Democratic convention this summer.

Moderate VP would be a 'disaster'

US Vice President Joe Biden (right) with former Secretary of State Hillary Clinton

Elsewhere in his remarks on Friday, Sanders said that a Democratic presidential ticket of Clinton and a “conservative or moderate” running mate would be a “disaster.”

“If Hillary Clinton were to win and Hillary Clinton were to bring onboard a conservative or moderate-type Democrat, I think politically that would be a disaster,” he said.