U.S media reports say Speaker of the House of Representatives Nancy Pelosi announced the House is launching a formal impeachment inquiry into President Trump, setting up a dramatic constitutional clash just over a year before the presidential election.

“Today I'm announcing the House of Representatives is moving forward with an official impeachment inquiry," Pelosi said at the Capitol late Tuesday afternoon, according CBS News.

The inquiry marks just the fourth time in American history a president has faced a viable threat of impeachment.

The speaker has reportedly long resisted calls from many progressive lawmakers to initiate impeachment proceedings against the president, but Democrats appear to have reached a breaking point over the administration's refusal to hand over a whistleblower complaint related to Mr. Trump's interaction with a foreign leader.

“This week, the president has admitted to asking the president of Ukraine to take actions which would benefit him politically,” Pelosi said.  “The actions of the Trump presidency revealed dishonorable facts of the president's betrayal of his oath of office, betrayal of national security and betrayal of the integrity of our elections.”

CBS News says the current furor stems from a call Mr. Trump made to the president of Ukraine in July, in which he admitted discussing Joe Biden in the context of fighting “corruption” in the country.  Mr. Trump and his allies, in particular personal attorney Rudy Giuliani, have accused Biden of pushing for the ouster of a Ukrainian prosecutor while he was vice president in order to benefit his son.  The prosecutor was widely seen as corrupt, and no evidence of wrongdoing by Biden has emerged.

In August, an anonymous member of the intelligence community filed a whistleblower complaint with the intelligence community inspector general, who determined it constituted an "urgent concern" requiring congressional notification under federal law.  However, after consulting with the Justice Department and White House, the acting director of national intelligence came to a different conclusion, and has refused to provide the complaint to congressional committees.

Pelosi said the administration's refusal to provide the complaint was a "violation of the law" that "undermine[s] both our national security and our intelligence."

One after another on Monday and Tuesday, Democrats from vulnerable House districts who had been resisting previous calls for impeachment reportedly came out in favor of initiating impeachment proceedings, citing concerns over Mr. Trump's potential pressuring of a foreign leader to investigate a domestic political opponent.