A second whistleblower has come forward with information about President Donald Trump's dealings with Ukraine, adding to the impeachment peril engulfing the White House and potentially providing new leads to Democrats in their unfurling investigation of Trump's conduct.

Attorney Mark Zaid, who represents both whistleblowers, said the second person has spoken to the intelligence community's internal watchdog and can corroborate information in the original whistleblower complaint. That document alleged that Trump pushed Ukraine's president to investigate Democratic presidential candidate Joe Biden's family, prompting a White House cover-up. Crucially, the new whistleblower works in the intelligence field and has "firsthand knowledge" of key events, Zaid said.

The emergence of the second whistleblower threatened to undermine arguments from Trump and his allies to discredit the original complaint. They have called it politically motivated, claimed it was filed improperly and dismissed it as unreliable because it was based on secondhand or thirdhand information.

A rough transcript of Trump's call with Ukrainian President Volodymyr Zelensky, released by the White House, has already corroborated the complaint's central claim that Trump sought to pressure Ukraine to investigate the Bidens. The push came even though there was no evidence of wrongdoing by the former vice president or his son Hunter, who served on the board of a Ukrainian gas company.

Text messages from State Department officials revealed other details, including that Ukraine was promised a visit with Trump if the government would agree to investigate the 2016 election and Ukrainian gas company Burisma – the outline of a potential quid pro quo.

Rep. Jim Himes, D-Conn., a member of the House Intelligence Committee, said word of a second whistleblower indicates a larger shift inside the government.

"The president's real problem is that his behavior has finally gotten to a place where people are saying, 'Enough,'" Himes said.

Democrats have zeroed in on the State Department in the opening phase of their impeachment investigation. The Intelligence, Oversight and Foreign Affairs committees have already interviewed Kurt Volker, a former special envoy to Ukraine who provided the text messages. At least two other witnesses are set for depositions this week: Gordon Sondland, the U.S. ambassador to the European Union, and Marie Yovanovitch, who was abruptly ousted as the U.S. ambassador to Ukraine in May.

Trump and his supporters deny that he did anything improper, but the White House has struggled to come up with a unified response. No administration officials appeared on the Sunday news shows to defend the president, while other Republicans focused mainly on attacking Democrats.