WASHINGTON (Reuters) - The U.S. Senate was poised on Wednesday to acquit President Donald Trump on charges of abuse of power and obstruction of Congress.

Vice President Mike Pence an Speaker of the House Nancy Pelosi (D-CA) listen as U.S. President Donald Trump delivers the State of the Union address to a joint session of the U.S. Congress in the House Chamber of the U.S. Capitol in Washington, U.S., February 4, 2020. REUTERS/Joshua Roberts

Here is a look at the road to his impeachment and trial.

MUELLER REPORT

In July 2017, two little-known Democrats in the House of Representatives make the first attempt to impeach Trump, basing it on investigations into Moscow’s interference in the 2016 U.S. elections.

They file formal charges known as articles of impeachment, alleging that Trump obstructed justice by firing FBI head James Comey to hinder the Russia investigation by then Special Counsel Robert Mueller.

After a two-year probe, Mueller finds insufficient evidence of a criminal conspiracy involving Trump and draws no conclusions on whether Trump obstructed justice.

Little comes of this impeachment effort. In 2019 House Speaker Nancy Pelosi, the leading Democrat in Congress, says Trump is “just not worth it.”

WHISTLEBLOWER

On Aug. 12, 2019, an anonymous intelligence official files a whistleblower complaint about a phone call between Trump and Ukrainian President Volodymyr Zelenskiy. According to a summary of the call later released by the White House, Trump asked Zelenskiy to investigate both former U.S. Vice President Joe Biden, a Democrat running for president in 2020, and a conspiracy theory that Ukraine, not Russia, was behind 2016 election meddling.

IMPEACHMENT INVESTIGATION

Amid further reports of Trump’s dealings with Ukraine, Pelosi on Sept. 24 accuses Trump of urging a foreign power to intervene in the 2020 election and announces an impeachment investigation in the Democratic-controlled House.

In testimony at televised hearings, U.S. diplomats and officials describe a pressure campaign organized by Trump’s lawyer, Rudy Giuliani, to get Ukraine to announce the Biden investigation. Trump’s freezing of $391 million in aid to Ukraine was part of the campaign, according to some testimony.

TRUMP’S DEFENSE

Trump says he was just trying to get Kiev to quash corruption and points to the fact that the Ukrainian investigations never happened as evidence that the July call was innocent. Republican lawmakers say that even if Trump’s conduct was not perfect, it does not belong in the category of “high crimes and misdemeanors” stipulated in the U.S. Constitution as a reason to impeach a president.

IMPEACHED

On Dec. 18 Trump becomes only the third U.S. president to be impeached when the Democratic-led House approves two articles of impeachment charging him with abuse of power and obstructing Congress.

THE TRIAL

The Senate puts Trump on trial in late January. The outcome seems in little doubt, as Republicans dominate the chamber where a two-thirds majority is needed to convict and remove the president. No Republican expresses a desire to find Trump guilty.

WITNESSES

The two parties tussle over Democrats’ attempts to introduce new witnesses and documents to the trial. They are especially keen to hear from former national security adviser John Bolton who had disparaged efforts by Trump allies to influence Ukraine as “a drug deal.”

THE BOLTON FACTOR

The New York Times reports that Bolton, a foreign policy hawk who has served previous Republican administrations, has written a book in which he says Trump told him he had wanted to continue freezing the aid to Ukraine to pressure Zelenskiy to help with investigations into Democrats, including Biden. The revelation contradicts Trump’s defense. It puts pressure on a handful of moderate Republican senators to join Democrats and vote to call Bolton and White House officials as witnesses. But a majority of senators vote to reject new witnesses.

TRUMP DEFENSE

Trump lawyer Alan Dershowitz says at the trial that senators cannot impeach a president for doing anything to try to win re-election if the president believes that to be in the public interest.

Democrats and constitutional experts severely criticize this defense, saying it gives U.S. presidents huge new powers not foreseen in the Constitution. Dershowitz says later his remarks were deliberately misinterpreted.