Donald Trump has been accused of attempts to pressure Ukraine for personal political gain.

A second whistleblower has come forward, this one with first-hand information about events that triggered an impeachment investigation into President Donald Trump for alleged abuse of power, the informant's lawyer said Sunday.

"I can confirm this report of a second #whistleblower being represented by our legal team," Mark Zaid said on Twitter. "They also made a protected disclosure under the law and cannot be retaliated against. This WBer has first-hand knowledge."

Zaid's co-counsel, Andrew Bakaj, said earlier that his firm and team "represent multiple whistleblowers" in the case accusing Trump of using the powers of his office to pressure Ukrainian President Volodymyr Zelensky to investigate political rival Joe Biden and his son Hunter.

With the crisis seeming to deepen by the day, it was unclear whether Bakaj was using "multiple" to refer to more than two whistleblowers. Typically, several officials would listen in on a call between the president and a foreign leader.

The existence of a whistleblower claiming first-hand knowledge would make it harder for the president and his supporters to dismiss the original complaint as hearsay, as they have repeatedly done.

Trump pushes back

Trump pushed back at the allegations in tweets Sunday, but did not mention the second whistleblower.

He repeated his assertions that Hunter Biden had been "handed $100,000 a month (Plus,Plus) from a Ukrainian based company, even though he had no experience in energy...and separately got 1.5 Billion Dollars from China despite no experience and for no apparent reason."

Media reports have said Hunter Biden was paid up to $50,000 a month as a member of the board of a Ukrainian gas company, Burisma.

Trump tweeted that "as president I have an obligation to end CORRUPTION, even if that means requesting the help of a foreign country or countries. It is done all the time."

In a back-and-forth on Twitter, Biden soon responded: "In my experience, asking a foreign government to manufacture lies about your domestic political opponent is not 'done all the time.'"

Trump also said that Biden, for months the leading candidate for the Democratic presidential nomination in 2020, should "hang it up."

No evidence has been found that either Biden did anything illegal.

In perhaps his strongest response yet, Joe Biden wrote in The Washington Post that Trump was "frantically pushing flat-out lies, debunked conspiracy theories and smears against me and my family, no doubt hoping to undermine my candidacy."

"It won't work, because the American people know me -- and they know him," Biden said in an op-ed article.

A bit unusually, Trump stayed in the White House on Sunday rather than traveling or playing golf.

No administration officials appeared on the Sunday television programs despite the critical nature of the moment.

But one Republican senator, Ron Johnson, told NBC's "Meet the Press" that Trump, in a conversation, had sharply rejected allegations he linked military aid for Ukraine to any effort to find dirt on the Bidens.

"When I asked the president about that," said Johnson, who serves on the Foreign Relations Committee, "he completely adamantly, vehemently, angrily denied it."

'Harassment'

Democratic-led House committees issued a series of subpoenas in the matter on Friday, including to the White House.

The impeachment saga began after the original whistleblower -- an intelligence official -- filed a formal complaint to the intelligence community inspector general about Trump's alleged pressuring of Zelensky.

A rough transcript of the phone call later released by the White House, as well as a series of text messages between US diplomats, appeared to corroborate the original complaint.

Democrats -- derided in a Trump tweet Sunday as "DONOTHINGDEMOCRATS" -- insist that they will continue pushing for action on key issues like health care and gun control even as the impeachment inquiry absorbs a huge amount of Washington's attention and energy.

But in an appearance on NBC, former CIA director John Brennan raised questions about the country's continuing stability under Trump.

Asked how the CIA might assess the stability of the United States -- as the agency does with other countries -- Brennan said: "We would look at it as a very corrupt government that is under the sway, right now, of this powerful individual who has been able to just corrupt the institutions and the laws of that country."

"I think there's a real question about the stability," he added.