Former US House Speaker John Boehner says it is very likely that former Secretary of State Hillary Clinton will withdraw from the presidential race due to her email scandal and be replaced by Vice President Joe Biden.

Addressing a conference in Las Vegas on Thursday, Boehner said that Democrats should turn to Biden to be their 2016 nominee.

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

Elsewhere in his remarks, Boehner slammed former Republican presidential candidate Ted Cruz. "Thank God that guy from Texas didn't win" the nomination, he said.

Boehner had called Cruz "Lucifer in the flesh" and a "miserable son of a ***.” "I got criticized by some satanic organization for giving Lucifer a bad name," he quipped.

After Donald Trump's commanding victory in Indiana's primary earlier this month, his remaining challengers, Cruz and John Kasich, both suspended their presidential bids, leaving the businessman tycoon on an uncontested path to the Republican nomination.

Boehner said anyone who thinks Trump will not win the White House in November should "just watch."

"This is going to be a presidential campaign like we've never seen before," he said.

Former US House of Representatives Speaker John Boehner (AFP photo)

Former Republican presidential candidate Ted Cruz (AFP photo)

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.

The Federal Bureau of Investigation (FBI) is likely to interview Clinton in the next few weeks about the issue. The FBI has already interviewed some of Clinton’s aides, including Huma Abedin, the vice chairwoman of Clinton's presidential campaign.

The nine-month FBI investigation into whether laws were broken as a result of the server kept in her New York state home has overshadowed Clinton's campaign.

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.

Donald Trump, the US Republican Party’s presumptive presidential nominee (AFP photo)

US Vice President Joe Biden (AFP photo)

Last week, a US federal judge in Washington said he may order Clinton to testify under oath about whether she used a private email server.

US District Court Judge Emmet Sullivan signed an order granting a request from the conservative watchdog group Judicial Watch to question six current and former US State Department staffers about the creation and purpose of the private email server.

Based on what might be discovered from those interviews, which are to be conducted over the next eight weeks, Sullivan says in his order an out-of-court testimony from Clinton "may be necessary."

That raises the possibility that Clinton could be ordered to testify in the middle of the US presidential race.

Clinton has come under fire for using her private email account and server. Critics, including Trump, say she endangered government secrets and evaded transparency laws.

According to American political analyst Scott Bennett, the belief that Clinton will crash and burn makes it certain that Biden will seek the nomination.

“She could be indicted for treason, misprision of felony, all sorts of confidential violations of emails and information from her role at the State Department,” Bennett, a former US Army psychological warfare officer in San Francisco, told Press TV last year.

“But more importantly than that all of those emails are slowly being exposed and what is being revealed is Hillary Clinton’s connection to the Union Bank of Switzerland and the Swiss bank financing the Wahabi/Salafi/terrorist regimes in the Middle East,” Bennett stated.

“Hillary Clinton was involved up to her neck in that along with Eric Holder and Lanny Breuer; so all of that is coming out,” he noted.