Most of Joe Biden's 2020 Democratic opponents are pushing for the impeachment of President Trump over the whistleblower complaint involving Ukraine but are remaining silent on the subject of Hunter Biden's activities in Ukraine.

That may change soon.

The former vice president bragged in 2018 that he threatened to withhold $1 billion in United States loan guarantees during a 2016 trip to Kyiv if Ukraine didn’t fire top prosecutor Viktor Shokin, who had investigated Ukrainian oligarch Mykola Zlochevsky, whose Burisma Holdings company paid Biden's son, Hunter, $50,000 a month to sit on its board from 2014.

Bernie Sanders deflected questions about whether Democrats would use this against Joe Biden in the 2020 race but did not say the subject was off limits.

“You guys write very perceptive articles on these things, and I’ll let you make that judgment,” he said in Iowa on Tuesday when pressed on whether the candidacy of Delaware's senator for 36 years had been weakened.

To some, the comment was reminiscent of the famous Francis Urquart utterance to a journalist in the original version of the television show House of Cards: "You might think that. I couldn't possibly comment."

Kamala Harris, who called on Trump to “leave Joe Biden alone” last weekend in Iowa, told journalists on Tuesday in Los Angeles it was up "to the voters to decide" the ramifications.

The Democratic debates present a ripe opportunity for Biden's Democratic rivals to frame it as a classic case of Washington insiderism and cronyism, according to Republican strategist Brent Littlefield. The facts remain potent even if inextricably linked to Trump's attempts to stong-arm the Ukrainian government into investigating allegations against Hunter Biden.

"If you strip everything away from the current political and media environment regarding the president and everything happening on Capitol Hill and just look at the basics, certainly Joe Biden's opponents could take advantage of the situation by just making the point that he was a lead person within the Obama administration on the issue of Ukraine and yet his own son was earning large sums of money from a company in Ukraine, which was apparently connected to the government," Littlefield told the Washington Examiner. "It's not much more complicated than that."

But pollster John Zogby recommended that White House hopefuls exercise caution, advocating for a more indirect approach if a campaign had Ukrainian dirt on the former vice president. He said staff members could share the information "with friendly or ambitious journalists" or have social media experts "plant ideas that can potentially go viral."

He said: "Ultimately, Biden is still a front-runner. He's going to need their support, they're going to need his support, whoever wins the nomination, so it's not wise to overtly anger him. Besides, it could blow up in their face. We don't have any evidence yet that this is sticking or hurting Joe Biden."

It might be wiser, he suggested, to wait for more damning information about Hunter Biden to emerge before salvos against Biden because attacks "could cause a free fall" for rivals who judged the moment wrongly.

"Look at what happened with Kamala Harris," Zogby said of the California senator. "Her numbers went up when she attacked Biden, but then they went right back down because she was seen then as a very ambitious person who would stop at nothing. It could backfire, especially if it's all defensible."

House Speaker Nancy Pelosi on Tuesday announced a formal impeachment inquiry into Trump after an intelligence community whistleblower came forward to complain about a July 25 telephone conversation between the president and Ukrainian leader Volodymyr Zelensky in which he pressured Zelensky to probe a matter related to Biden.