ROCHESTER, New Hampshire — US President Donald Trump added muscle to the legal team defending against an impeachment investigation led by congressional Democrats on Wednesday, after 2020 re-election rival Joe Biden called for the first time for his impeachment.

Biden, who is at the center of a controversy over Trump's dealings with Ukraine that led the Democratic-led US House of Representatives to open an impeachment inquiry, had previously refrained from making an outright plea for impeachment.

Trump continued to paint the probe as a partisan smear, and accused the US intelligence officer who filed the whistle-blower complaint that sparked the furor of having political motives. He also added former US Representative Trey Gowdy, best known for his investigations of the administration of Trump's Democratic predecessor, to his outside legal team.

During a campaign stop in New Hampshire, Biden, the Democratic front-runner to face Trump in next year's presidential election, took the gloves off.

"With his words and his actions, President Trump has indicted himself. By obstructing justice, refusing to comply with a congressional inquiry, he's already convicted himself," Biden said. "In full view of the world and the American people, Donald Trump has violated his oath of office, betrayed this nation and committed impeachable acts."

"To preserve our Constitution, our democracy, our basic integrity, he should be impeached."

Trump fired back on Twitter.

"So pathetic to see Sleepy Joe Biden, who with his son, Hunter, and to the detriment of the American Taxpayer, has ripped off at least two countries for millions of dollars, calling for my impeachment — and I did nothing wrong," Trump wrote.

The House began impeachment proceedings against Trump last month over his attempts to have Ukraine's president investigate Biden and his son Hunter Biden, who was on the board of a Ukrainian gas company.

Despite Trump's allegations, which he made without evidence, that Biden engaged in improper dealings in Ukraine, there are few signs the controversy has damaged the Democratic former vice president's 2020 prospects.

Public opinion polls, including those taken by Reuters/Ipsos, have shown Biden's support remaining relatively stable.

Trump on Wednesday again described the inquiry as a partisan attack.

"It turns out that the whistle-blower is a Democrat, strong Democrat, and is working with one of my opponents as a Democrat," Trump told reporters.

Lawyers for the whistle-blower responded in a statement, denying that political factors had influenced the complaint.

"Our client has never worked for or advised a political candidate, campaign or party," they said in a statement. "Our client has spent their entire government career in apolitical, civil servant positions."

The day after the White House declared its refusal to cooperate with the impeachment probe, Trump added that he would respond if House Democrats "give us our rights."

The addition of Gowdy to Trump's legal team marked a pivot from late September, when outside lawyer Jay Sekulow said there were no plans to beef up the legal team. Sekulow announced Gowdy's hiring on Wednesday.

The three congressional committees leading the inquiry were working on final arrangements on Wednesday to interview the whistle-blower.

The State Department this week abruptly blocked the US ambassador to the European Union, who had been in touch with Ukrainian officials on Trump's behalf from speaking to the inquiry.

The investigation is focused on whether Trump used almost $400 million in congressionally approved aid to Ukraine as leverage to pressure Ukrainian President Volodymyr Zelenskiy to begin an investigation of the Bidens.

Trump has defended the July 25 phone call to Zelenskiy.

Most Democrats want to impeach Trump, even if that means weakening their party's chances of winning back the White House in 2020, according to a Reuters/Ipsos opinion poll.

The poll, conducted on Monday and Tuesday, found that 55 percent of Democrats said their party leaders should press ahead with impeachment even "if it means a lengthy and expensive process that could weaken their chances of winning the presidency in 2020." — Reuters