President Trump’s former special envoy to the Ukraine appeared behind closed doors before lawmakers from three House committees Thursday as Democrats stepped up their impeachment inquiry over whether Trump abused the power of his office.

Kurt Volker, who resigned his post last week, was among the figures mentioned in a whistleblower complaint involving a July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelensky for a “favor” just days after Trump had frozen military aid to the country fighting Russia-backed separatists in its eastern provinces. Trump asked Zelensky to investigate former Vice President Joe Biden, a leading contender for the 2020 Democratic presidential nomination.

Volker gave Rudy Giuliani, Trump’s personal lawyer, entrée to officials in Kyiv.

Trump has defended that call as “perfect” and Thursday also invited China to investigate Biden and his son, Hunter.

“I would think that if they were honest about it, they’d start a major investigation into the Bidens,” Trump told reporters at the White House.

“I would say that President Zelensky, if it were me, I would recommend they start an investigation into the Bidens because nobody doubts they weren't crooked."

Trump accused Biden of "scamming" other countries and said he thinks "Biden is going down."

Before departing to sign an executive order in Florida, Trump added: “China should start an investigation into the Bidens.”

Federal Election Commission Chairwoman Ellen Weintraub Thursday drew attention to a June 13 statement in which she pointed out it is illegal for candidates to solicit help from a foreign government.

Trump also added to his tweetstorm on the issue, posting a Breitbart poll indicating nearly all respondents want House Intelligence Committee Chairman Adam Schiff, who is leading the impeachment investigation, to resign. Trump has been bashing Schiff as “shifty,” a “lowlife” and a “fraud” for days.

Concern was rising Thursday over the accuracy of the transcript of the July 25 call released by the White House. The document itself is labeled as a memorandum assembled from the notes people took during the call while Trump Wednesday said it was an “exact word-for-word transcript,” despite it being riddled with ellipses, indicating words had been left out.

The reconstructed transcript is just five pages, but the White House has said the call lasted 30 minutes. The Washington Post reported its analysis of the transcript indicates it is about half as long as similar calls with other foreign leaders.

Meanwhile, Biden vowed to fight any efforts “to destroy me … [or] destroy my family.”

“Let me make something clear to Trump and his hatchet men and the special interests funding his attacks against me: I’m not going anywhere,” Biden said in a speech in Reno, Nevada. Biden called Trump a “bully” who is afraid of “just how badly I would beat him next November.”

An Emerson University poll of 837 Ohio voters released Thursday indicated all three leading Democratic presidential candidates would Trump in head-to-head contests by 4% to 6%. Both Biden and Sen. Bernie Sanders, who suspended his campaign this week following a heart problem, would beat Trump 53% to 47%, and Sen. Elizabeth Warren would win 52% to 48%.

A Monmouth University poll of more than 1,000 voters nationally released Wednesday indicated 57% of those queried would like to see someone other than Trump win the 2020 presidential election.