Former Vice President Joe Biden took a shot at President Trump on Sunday, calling him “unhinged” amid Trump’s response to the impeachment inquiry and the president’s own allegations of wrongdoing by Biden and his son.

Speaking at a Service Employees International Union (SEIU) summit, the 2020 presidential primary contender continued his strategy of pushing back against Trump’s allegations and what his campaign has called the president’s “unprecedented abuse of power.”

"Have you ever seen -- rhetorical question -- a president ever so unhinged as this guy is?" the Democrat asked Sunday.

While Biden and Trump have been at odds long before the former vice president entered the 2020 presidential race, the animosity between the two has heated following the whistleblower complaint that claimed Trump pressured Ukrainian President Volodymyr Zelensky to investigate Biden and his son Hunter, and threatened to withhold military aid if the Ukrainians did not follow through.

TRUMP FIRES BROADSIDE AT 'PILLAGING' BIDENS, CLAIMS UKRAINE PUSH WAS ONLY ABOUT 'CORRUPTION'

Trump has accused Hunter Biden of improper activity related to his business dealings with energy company Burisma Holdings and claimed his father used his office to pressure the Ukrainian government into firing top prosecutor Viktor Shokin, who was investigating Burisma.

The former vice president has acknowledged publicly that he pressured Ukraine to fire Shokin but denied it had any relation to his son’s business dealings in the country. Biden said his actions were in line with pressure from other foreign leaders to fire the prosecutor.

“We weren’t pressing Ukraine to get rid of a tough prosecutor. We were pursuing Ukraine to replace a weak prosecutor who wouldn’t do the job,” Biden said this past Wednesday of an Obama administration policy supported by other Western governments that viewed Shokin as incompetent or perhaps corrupt.

The U.S. also had support internationally for its opposition to Shokin. Ukrainian officials, including Shokin’s successor, have said since that the Bidens were never implicated in any wrongdoing.

Biden’s “unhinged” comment came amid a challenging weekend for the Trump administration as reports emerged that a second whistleblower – this one with firsthand knowledge of the phone call between Trump and Zelensky – has spoken to the intelligence community’s inspector general.

The first whistleblower was subjected to criticism from Trump and his supporters based on an apparent lack of direct knowledge of the president’s conversation with Zelensky, as well as what Trump has called an inaccurate description of the call.

The new whistleblower also faced the ire of Trump, who has been even more active on social media over the weekend than normal. Targets of his social-media attacks included House Intelligence Committee Chairman Adam Schiff, D-Calif., and Republican Sen. Mitt Romney of Utah.

“The first so-called second hand information “Whistleblower” got my phone conversation almost completely wrong, so now word is they are going to the bench and another “Whistleblower” is coming in from the Deep State, also with second hand info, Trump tweeted. “Meet with Shifty. Keep them coming!”

Biden held a nine-point advantage over Trump in the crucial swing state of Wisconsin, according to a Fox News poll out Sunday.

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Biden bested Trump by nine points (48-39 percent) and had the only lead outside the poll’s margin of error. Bernie Sanders was up by five points over Trump (45-40) and Elizabeth Warren had a four-point edge (45-41). In each case, the remaining 13-14 percent of undecided/third-party voters was enough to shift the race either way.

Trump received 41 percent support or less in each of the matchups -- well below the 47 percent he received when he won Wisconsin in 2016. At the same time, none of the Democrats hit 50 percent.

Fox News' Dana Blanton, Adam Shaw and Ronn Blitzer contributed to this report.