EXCLUSIVE: WEST LEBANON, N.H. — When it comes to Sen. Bernie Sanders, former President Trump is no laughing matter.

The longtime independent senator from Vermont, progressive champion and two-time runner-up in the Democratic presidential primaries is on a two-day swing this weekend in neighboring New Hampshire as well as Maine to campaign on behalf of Vice President Kamala Harris to make sure the GOP presidential nominee doesn’t return to the White House.

“Trump cannot get elected. We’ve got to do everything we can to make sure that does not happen,” Sanders told a crowd of supporters during his first stop Friday in New Hampshire, a key swing state in presidential elections.

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But minutes earlier, Sanders briefly broke out in laughter when asked in a national exclusive interview with Fox News about comments from Trump this week arguing that Harris — who has replaced President Biden at the top of the Democrats’ 2024 national ticket — is more liberal than the Vermont senator.

Trump over the past week has worked to define Harris, a former San Francisco district attorney and California attorney general, as an ultra-liberal, pointing to her record in the U.S. Senate and as vice president.

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Speaking to a packed arena in Charlotte, North Carolina, on Wednesday, Trump charged that Harris was the “most incompetent and far-left vice president in American history… She is a radical left lunatic who will destroy our country if she ever gets the chance to get into office.” 

And mentioning Sanders, Trump argued that Harris is “more liberal than Bernie Sanders. Can you believe it?”

Sanders, responding, said, “I would hope that when he said, ‘Can you believe that?,’ people said no.”

“It’’s not true. Once again, Trump is lying,” Sanders emphasized. “Let me just simply say that for better or for worse, Kamala Harris is not more progressive than I am.”

During his Fox News interview and later at his event, Sanders took aim at Trump, who two months ago was convicted of 34 felony counts in the first criminal trial of a former or current president in the nation’s history.

“This is the most important election, I think, in our lifetimes. I will do everything that I can to see that Donald Trump is defeated,” the senator stressed.

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Sanders argued that “the American people will not and cannot accept a president who is a pathological liar, somebody who believes that women should not be able to control their own bodies, somebody who in the midst of massive heatwaves thinks climate change is a hoax and somebody who actually does not believe in democracy, has not said that he will accept those election results if he loses. So, for all of those reasons, Trump must be defeated.”

Sanders is campaigning on behalf of Harris, but he hasn’t formally endorsed the vice president.

“I think if the vice president is to win this election, and obviously I want her to win, I think she has to start talking about issues of relevance to the working class of this country, because there are tens of millions of people who are really hurting,” Sanders explained. “They want to know what the next president is going to do for them, and I hope very much that Vice President Harris will make that clear.”

“The path towards victory is to talk about issues that are relevant,” he reiterated.

Asked what Harris specifically needs to detail, Sanders said, “I hope that the vice president will be talking about the need to substantially lower prescription drug costs… the need to have tax reform so the wealthiest in this country start paying their fair share of taxes, so we can greatly expand child care and affordable housing in this country, and I think we’ve got to be very strong on the issue of climate change and make it clear that we’re going to transform our energy system away from fossil fuel if we’re going to save this planet for future generations.”

Sanders said that Harris’ choice of a running mate — which is expected to come in the next two weeks — will be a signal of whether she will project a progressive agenda as she runs for the White House.

“I think it will, and I hope very much she looks at one of the many progressive people who are out there who I think would do a good job as vice president,” the senator said.

Sanders was making his swing through New Hampshire and Maine less than a week after President Biden suspended his 2024 re-election rematch with Trump. Biden made his move amid mounting pressure from within the Democratic Party for him to drop out after a disastrous performance in last month’s first presidential debate with Trump. 

The embattled president’s immediate backing of Harris ignited a surge of endorsements of Harris by Democratic governors, senators, House members and other party leaders. By Monday night, the vice president announced that she had locked up her party’s nomination by landing the backing of a majority of the nearly 4,000 delegates to next month’s Democratic National Convention. On Friday morning, former President Obama and former first lady Michelle Obama were among the final major party leaders to endorse the vice president.

Harris has also hauled in a staggering $129 million in fundraising since Biden’s announcement, her campaign touted on Thursday morning. 

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Republicans charge that the process has been anything but democratic — and they point to Biden’s own words.

Before dropping out, the president had repeatedly cited the 14 million votes he won in this year’s Democratic presidential primaries as a reason he should stay in the 2024 race.

“The voters — and the voters alone — decide the nominee of the Democratic Party,” he emphasized in a letter on July 8. “Not the press, not the pundits, not the big donors, not any selected group of individuals, no matter how well intentioned.”

Trump, at his rally in Charlotte on Wednesday, called the switch at the top of the Democrats’ national ticket “an undemocratic move.” 

“These are nasty people, the Democrats,” Trump argued.

And Republican Sen. Tom Cotton of Arkansas claimed in a social media post this week that “Joe Biden succumbed to a coup by Nancy Pelosi, Barack Obama, and Hollywood donors, ignoring millions of Democratic primary votes.”

But Sanders, who argued during his marathon 2016 Democratic presidential primary battle against eventual nominee Hillary Clinton that the party was working against him, doesn’t buy the GOP criticism.

“These are extraordinary times and the Democrats had to move very quickly,” Sanders said. “So I think that given the reality that Biden dropped out and having a Democratic convention coming, I think what happened is she announced her candidacy, she rallied the support she needs, and I think that’s fine.”

Get the latest updates from the 2024 campaign trail, exclusive interviews and more at our Fox News Digital election hub.