(Reuters) - U.S. congressional Democrats are pursuing a fast-moving impeachment inquiry against President Donald Trump, focused on whether he abused his power by pressuring Ukraine to launch an investigation against a key political rival.

Democrats allege Trump temporarily withheld $391 million in U.S. military assistance to Ukraine as a way to pressure the country into investigating former Vice President Joe Biden -- a possible 2020 election challenger. A senior White House official has said the aid was linked instead to a request that Ukraine look into a debunked conspiracy theory about foreign interference in the 2016 U.S. presidential election, involving a Democratic Party computer server.

Some key events related to the probe follow:

** MAY 20: Trump recalls U.S. Ambassador to the Ukraine Marie Yovanovitch, who was overseeing a U.S. anti-corruption policy to promote the rule of law in the country, from her post in Kiev.

** MAY 23: Trump orders administration officials to work with his personal lawyer, Rudy Giuliani, on Ukraine. Giuliani, a private citizen, was trying to persuade Ukraine to investigate allegations of corruption Trump leveled against Biden -- a candidate for the 2020 Democratic presidential nomination -- and his son, Hunter.

The White House then places three officials -- Trump donor and U.S. Ambassador to the European Union Gordon Sondland, Energy Secretary Rick Perry and Trump’s then-special envoy to Ukraine, Kurt Volker -- in charge of Ukraine policy. The trio, who nickname themselves “the three amigos,” sideline career diplomats.

** JULY 10: Misgivings about the direction of Ukraine policy and Giuliani’s role boil over at the White House, when then-national security adviser John Bolton objects to an effort by Sondland and Giuliani to pressure Ukraine. He directs an aide to alert White House lawyers to their actions.

Bolton describes Giuliani as “a hand grenade who’s going to blow everybody up,” the aide, Fiona Hill, says in testimony reported in the U.S. media. Sondland raises the matter of investigations at a meeting, according to Hill. She and others take that to be a reference to the Bidens.

** JULY 25: Trump personally presses Ukrainian President Volodymyr Zelenskiy in a phone call to investigate Biden and his son over the son’s prior role as a director of a Ukrainian gas company. Trump also asks the Ukrainian to investigate the conspiracy theory about the 2016 U.S. election.

** AUG. 12: A career U.S. intelligence officer delivers a whistleblower complaint about the July 25 phone call to the chairmen of the intelligence committees of the Senate and House of Representatives.

** SEPT. 9: U.S. diplomats discuss via text messages concerns that military assistance to Ukraine was being withheld as a way to pressure the country into cooperating on the Biden probe. Sondland says Trump wanted to withhold the aid over concerns about corruption in Ukraine and denies there was a “quid pro quo” - a Latin phrase meaning something for something - of any kind.

** SEPT. 25: The Justice Department releases a summary of the Trump-Zelenskiy call. Trump and Zelenskiy meet in person for the first time at a U.N. gathering in New York.

** SEPT. 26: The House Intelligence Committee releases an unclassified version of the whistleblower complaint.

** OCT. 8: The White House refuses to cooperate with what it calls an “illegitimate, unconstitutional” impeachment inquiry, telling House Democrats in a letter that Trump and his administration cannot participate with the probe while fulfilling their “duties to the American people, the Constitution, the Executive Branch and all future occupants of the Office of the Presidency.”

The administration attempt to block witness testimony soon crumbles, as a succession of current and former foreign affairs officials appear for depositions in the U.S. Capitol.

** OCT 10: Two businessmen who helped Giuliani - Ukraine-born Lev Parnas and Belarus-born Igor Fruman - are arrested by U.S. authorities for their alleged involvement in a scheme to illegally funnel money to a pro-Trump election committee and political candidates.

** OCT 17: Sondland tells congressional investigators that Trump himself had told him repeatedly that there was no quid pro quo. That same day, White House acting Chief of Staff Mick Mulvaney tells a news conference that the aid was linked to Trump’s request for an investigation involving the 2016 U.S. presidential election and a Democratic National Committee server alleged to be in Ukraine. Mulvaney later contradicted himself in a statement from the White House that ruled out a quid pro quo.