Former Vice President Joe Biden, in an interview Tuesday in New Hampshire, said House Democrats probing allegations that President Trump obstructed justice are getting stonewalled and impeachment proceedings against the president “could come up very quickly.”

And Biden said there may be “no alternative” but to move toward an impeachment hearing as congressional Democrats vow to dig into the findings from Special Counsel Robert Mueller’s Russia report.

The comments amount to some of Biden's strongest language to date on the controversial impeachment issue, as many 2020 rivals urge proceedings to begin without delay. The clear front-runner in the race for the Democratic presidential nomination, Biden made the comments Tuesday in an interview following a campaign event in the first-in-the-nation primary state of New Hampshire.

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The former vice president also addressed the attacks he's been weathering of late from some rivals for the Democratic nomination.

“I’m not going to go down to anybody else’s level when they start attacks,” he said, when asked about the jabs. But he added "I’ll set the facts straight” and he specifically pushed back against criticism from a top rival – Sen. Bernie Sanders of Vermont – over his big-dollar fundraisers.

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Biden was in New Hampshire as it emerged that the White House had instructed two former aides, Hope Hicks and Annie Donaldson, to defy congressional subpoenas that sought documents related to allegations that Trump obstructed justice. It was the latest case of the White House pushing back against investigators from the Democrat-controlled House.

Biden said House Democrats are getting stonewalled and argued they have a “responsibility to move if in fact they are unable to get the data that is totally within their power to be able to subpoena before the Congress to make a judgment.”

He added: “And if they stonewall that, the only other constitutional avenue for them is impeachment.”

One of his top rivals in the race, Vermont Sen. Bernie Sanders, last week called for the House to begin impeachment proceedings.

“I believe the Judiciary Committee should begin impeachment inquiries,” Sanders said at a rally in Nevada. “That is inquiries, not impeachment, to determine whether or not Trump has committed impeachable offenses.”

Other 2020 candidates have explicitly called on Congress to move ahead with impeachment now. Sen. Cory Booker, D-N.J., said there is a "legal and moral obligation to begin impeachment proceedings immediately."

Sen. Elizabeth Warren, D-Mass., described Mueller's statement as "an impeachment referral," and said that Congress should act on it.

Sen. Kamala Harris, D-Calif., also compared Mueller's remarks to an "impeachment referral," and said, "We need to start impeachment proceedings. It's our constitutional obligation."

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Democrats in the House are suggesting they believe the party will eventually try to impeach Trump. South Carolina Rep. Jim Clyburn, the third-highest-ranking Democrat in the House, said in an interview on CNN Sunday that he believes impeachment proceedings ultimately will be launched against Trump at some point in the future. He suggested Democrats are already laying the groundwork in Congress, though later seemed to walk his comments back.

In a radio interview last week, House Judiciary Committee Chairman Jerry Nadler, D-N.Y., whose committee would lead impeachment proceedings, said “there certainly is” justification for Congress to begin impeachment proceedings against Trump, but said it was critical for the American public to be on board before launching the process.

Mueller's statement has put renewed pressure on House Speaker Nancy Pelosi, who has resisted the calls and questioned the political wisdom of moving forward. Pelosi has been non-committal about impeachment and said last week, "Many constituents want to impeach the president. But we want to do what is right and what gets results."

Even if they impeach, Democrats do not appear to have the votes in the Senate – where a supermajority is required and Republicans have control – to remove Trump from office.

Biden’s appearance in New Hampshire comes three days after he was indirectly attacked by some of his rivals who were campaigning in California.

With Biden 2,500 miles away – speaking to the Human Rights Campaign annual gala in Ohio – several of the 14 Democratic presidential candidates attending the California Democratic Party’s annual convention took aim at the former vice president.

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The verbal fireworks in San Francisco were the latest sign that the left wing of the Democratic Party is in no mood to see Biden – derided by many of them as a centrist establishment Democrat – become the party's standard-bearer in 2020

Among those directly jabbing at Biden were Sanders, Warren, and South Bend, Ind., Mayor Pete Buttigieg.

“We will not defeat Donald Trump unless we bring excitement and energy into the campaign,” declared Sanders, taking some of his strongest shots to date at Biden, without naming him.

“We cannot go back to the old ways,” argued Sanders, the independent senator from Vermont who’s making a second straight run for the Democratic nomination.

Responding to those critiques, and other sniping against Biden’s claim that Republicans would be amenable to compromise with Trump out of the White House, the former vice president told the audience in Berlin that “I’m not going about going back to the past. I’m talking about avoiding a terrible future if we do not figure out how to make this work. The Republicans are my opponents, they’re not my enemy.”

And he warned “there’s people who say that you can’t work with the other side. Well, if that’s the case, prepare your children for a totally different U.S. A totally different world. I don’t believe it.”

In his interview, Biden said, “I think the worst thing we could do is get into a match where we’re going after each other in the Democratic Party. So I’m going to try my best not to be negative relative to my opponents.”

Asked if he’ll respond to attacks during the upcoming Democratic primary debates, Biden said in the interview that “It depends. Just like I’m not going to go down to Trump’s level when he starts his attacks, I’m not going to go down to anybody else’s level when they start attacks.”

SANDERS SLAMS BIDEN FOR SWANY FUNDRAISERS

But he said he would set the record straight and pushed back against repeated jabs from Sanders over Biden’s top-dollar fundraisers.

“The stuff about Biden with the big contributors. We’ve had over 300,000 individual contributors. Average contribution of these is under $200,” Biden revealed as he touted his grassroots on-line contributions.“I’ve taken money as high as $2,800 from people. But my lord, this idea of Biden’s the big donor guy, come on. I’ve eschewed any relationship with any PACs.”

And taking aim at some of his rivals, he added “I’ll respond to assertions. I’m not likely to go and point out what they’re doing, which is sometimes different than what they say.”

Fox News’ Ronn Blitzer contributed to this report.