Joe Biden is dismissing calls from President Trump and his allies that Biden testify during an impeachment trial in the Senate, saying any effort to compel his testimony should be viewed as part of a strategy to distract from the president's conduct.

"No, I'm not going to let you take the eye off the ball here. Everybody knows what this is about," the former vice president told NPR when asked if he would cooperate with a subpoena. "This is a Trump gambit he plays. Whenever he's in trouble he tries to find someone else to divert attention to."

Asked a second time if he would comply with a subpoena, Biden said, "No, I will not yield to what everybody is looking for here. And that is to take the eye off the ball." He added, "no one has a proved one scintilla of evidence that I did anything other than do my job for America as well as anybody could have done it."

Biden was referring to attacks from Trump directed at his son, Hunter Biden, who took a lucrative job as a board member of a Ukrainian energy company when the elder Biden was vice president and oversaw Ukraine policy for the Obama White House.

The Bidens' dealings in Ukraine became a top focus of Trump and his Republican allies, and House Democrats are now preparing to impeach the president over allegations that he abused his office by pressuring Ukrainian authorities to open an investigation into the Bidens and other Democrats while crucial military aid was on hold.

Hunter Biden has said that joining the board of gas company Burisma Holdings was the result of "poor judgement," since it fueled his father's critics.

Last Thursday, President Trump tweeted that the former vice president and his son would both be among those who would testify in a Senate impeachment trial. "We will have Schiff, the Bidens, Pelosi and many more testify, and will reveal, for the first time, how corrupt our system really is," the president wrote.

State Department official George Kent testified in the impeachment probe that while he didn't see any improper behavior by Biden, he did warn his staff in early 2015 about possible perceptions of a conflict of interest, and that he was told the then-vice president could not deal with the issue at the time since his other son Beau was in the final months of his life suffering from brain cancer.

"Nobody warned me about a potential conflicts of interest. Nobody warned me about that," Biden said. He added, "They should have told me."

Despite the optics, the former vice president underscored that his son did nothing improper.

"The appearance looked bad and it gave folks like Rudy Giuliani an excuse to come up with a Trumpian kind of defense, while they were violating the Constitution," Biden said.

Campaign clashes

In a wide-ranging interview with NPR, while Biden was campaigning in Iowa, the Democratic frontrunner said his long career in politics makes him best positioned to restore America's standing on the world stage. Biden said his health-care plan is more "realistic" than policies proposed by some other Democratic contenders.

And he defended calling an Iowa voter a "damn liar" after the voter, who identified himself as a retired farmer, questioned whether the former vice president was too old the run for president and challenged his family's dealings in Ukraine.

Biden name-calling a voter drew comparisons to Trump's blunt and sometimes coarse style, a parallel that Biden strenuously rejected.

Trump, he said, "makes fun of people. He belittles people. He lies. I don't do any of those things. Period. The fact of the matter is this guy stood up, and he was, in fact, lying. And I just pointed out, 'You're a liar.' It's a fact: he lied. Period," Biden said.

When asked about the health care proposals of some of his opponents, like Sens. Elizabeth Warren and Bernie Sanders, who each support "Medicare for All" plans, Biden sounded skeptical.

"Do you think it's remotely possible to raise three point five trillion dollars a year, more than every single penny we spend on every single thing in the federal government on a yearly basis, without raising taxes on the middle class? If you answer that question, then I'm ending the interview because you know it's not true," Biden said. "You gotta raise taxes."

He did not name any of other Democratic candidates by name, but he said his opponents are "bright" and "honorable people," though argued that some of the bold proposals sparking policy debates do not have a real shot of ever becoming law in America.

"All the things we're talking about, none of it matters, even if you defeat Trump, unless you can pass it," Biden said.

What's driving the Biden campaign

Biden's decades in Democratic politics have allowed him to meet leaders around the world, experience that he said should give him an edge on foreign policy matters.

"I know all of these world leaders, even the ones that we don't like very much like Putin. I know him. He knows I know who he is, and he knows who I am. There's no misunderstanding about who we are. And it's really important," Biden said.

Biden continued: "Think about this. If we don't figure out how to bring the world together again. Reassure our allies, making sure that, that those who are opponents know we understand what they're about and we're going do something about it. Where do we go? What happens in five years? If this if the president's reelected, you think there'll be a NATO? Do you think there will be national security arrangements we have with European countries? What do you think? I don't think it's possible. I think they'll be gone."

While Biden has for months maintained a commanding lead in national polls, he has struggled to capture large numbers of younger and more liberal members of the Democratic Party.

When asked what might explain the apparent enthusiasm gap, Biden shot back: "There isn't an enthusiasm gap! What the hell are you talking about?"

He added: "Come with me on college campuses. You don't see that." However, while Biden holds a lead in national polling among Democratic voters, he consistently trails Sanders among voters under 30.

Biden's decision to run for president for the third time in his political career is a direct response to President Trump, who he said has placed the U.S. in "a deep, deep, deep hole."

"This is absolutely a guy who is damaging us around the world. And the issues that need to be dealt with now are issues that have been in my wheelhouse my whole life," Biden said. "How can I walk away? I'm not joking. I'm not saying I'm a savior. But they're the issues that were my wheelhouse my whole life."

Watch the full conversation here.

This interview was edited and produced by NPR's Eric Marrapodi, Victoria Whitley-Berry, and Bridget De Chagas.

NPR’s Eric Marrapodi, Victoria Whitley-Berry, and Bridget De Chagas contributed to this report.