Washington (CNN) Vice President Joe Biden offered effusive praise for Democratic presidential candidate Sen. Bernie Sanders Monday, lauding Hillary Clinton's chief rival for doing a "heck of a job" on the campaign trail and praising Sanders for offering an authentic voice on income inequality.

And while Biden said Democrats had a slate of "great candidates" running for president, he suggested Clinton was a newcomer to issues like the growing gap between rich and poor.

"Bernie is speaking to a yearning that is deep and real. And he has credibility on it," Biden said during an interview with CNN chief political analyst Gloria Borger.

"It's relatively new for Hillary to talk about that," Biden continued, acknowledging that Clinton has "come forward with some really thoughtful approaches to deal with the issue" of income inequality.

"Hillary's focus has been other things up to now, and that's been Bernie's -- no one questions Bernie's authenticity on those issues," he said.

Clinton and Sanders are locked in tight races in both Iowa and New Hampshire, which hold the nation's first nominating contests in less than a month's time. That's a distant cry from the start of the race, which saw Clinton an overwhelming favorite among Democrats.

The tightening in polls prompted Sanders, a Vermont senator who identifies as a Democratic Socialist, to jab at Clinton's campaign as being in "serious trouble" during a campaign stop in Iowa Monday.

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Biden expressed little shock that Sanders was drawing ample support among Democrats, claiming that Sanders' self-identification as a socialist mattered little to his party's voters.

"If Bernie Sanders never said he was a democratic socialist, based on what he's saying people wouldn't be calling him a democratic socialist," he said, claiming Clinton entered the race with an "awful high bar for her to meet."

"I never thought she was a prohibitive favorite," he said. "I don't think she ever thought she was a prohibitive favorite. So I think it's, you know, everything's sort of coming down to Earth."

Sanders has sufficiently come around on the issue of gun control, Biden said, even as the Clinton campaign continued to launch withering criticism of Sanders' past vote allowing legal immunity for gun manufacturers whose products are involved in fatal shootings. President Barack Obama recently wrote in an op-ed he wouldn't campaign for any candidate that doesn't support "common-sense gun reform."

"What Bernie Sanders has to do is say that the Second Amendment says -- which he has, of late -- the Second Amendment says you can limit who can own a gun, that people who are criminals shouldn't have guns," he said. "People who are schizophrenic and have mental illnesses shouldn't own guns. And he has said that."

Biden, who spent much of last year contemplating a third presidential bid, announced in October he wouldn't pursue the Democratic nomination, saying window had closed on jumping into the race as he and his family grieved the June death of his eldest son Beau.

He insisted Monday that was the "absolute right decision for my family" and offered little indication his mind could be changed given outcomes in the upcoming primary season.

"I don't think there's any door to open," he said when asked if he was closing the door fully on a 2016 bid, adding that even if Sanders ekes out victories in the early voting states, he still confronts an uphill climb to the nomination.

"Even if Hillary loses votes -- I've thought this through -- it's a long way to go in the nomination," he said, calling Sanders' prospects of winning South Carolina -- which holds its primary after New Hampshire -- "tough sledding."

Biden offered little praise for the leading Republican in the presidential race, saying Donald Trump would likely come to wish he hadn't used such disparaging language in this year's context.

"If Donald Trump gets the nomination and wins the election, if he's as smart as I think, he's going to regret having said the things he's said and done," Biden said. He suggested Trump enroll in "a couple crash graduate courses before he started to try to exercise the role of president."

But Biden did reveal a longing for the campaign trail, a setting he occupied regularly for four decades as a U.S. senator, and later as vice president, but will no longer experience again as a candidate.

"I wish sometimes that I was able to be out there making the case why there's such reason for optimism for this country," he said. "But I'm able to do that as vice president."

He still thinks about his late son "all the time," Biden said during the interview, remembering Beau Biden as "the most fastidious, honorable, straight guy."

The onetime prospect that Beau might have to resign as Delaware's attorney general, due to potential cognitive complications after a stroke, prompted a striking moment between Obama and the vice president, Biden recalled.

Describing a conversation during one of their weekly lunches, Biden said he was concerned about caring for Beau's family without his son's salary.

"I said, 'but I worked it out.'" Biden recalled telling Obama. "I said, 'but -- Jill and I will sell the house and be in good shape.'"

"And he got up and he said, 'Don't sell that house. Promise me you won't sell the house,'" Biden continued, speculating Obama would be "mad" he was retelling the story.

"He said, 'I'll give you the money. Whatever you need, I'll give you the money. Don't, Joe -- promise me. Promise me.' I said, 'I don't think we're going to have to anyway.' He said, 'promise me,'" Biden said.