WASHINGTON (Reuters) - The impeachment trial of President Donald Trump begins in earnest in the U.S. Senate on Tuesday, after he was formally charged by the Democratic-controlled House of Representatives on Dec. 18 with “high crimes and misdemeanors.”

FILE PHOTO: U.S. President Donald Trump talks to reporters as he departs for travel to New Orleans, Louisiana from the South Lawn of the White House in Washington, U.S., January 13, 2020. REUTERS/Leah Millis/File Photo

The Republican president, who is expected to be acquitted by the Republican-controlled Senate, says he is innocent of the charges.

Here is the Democrats’ case for removing Trump from office, and the Republican counter-argument.

THE CHARGES

In their articles of impeachment here , Democrats charge that Trump abused his power as president by pressuring the government of Ukraine to help him win re-election. They accuse the president of endangering the U.S. Constitution, jeopardizing national security and undermining the integrity of the 2020 election.

They also charge Trump with obstruction of Congress for refusing to turn over records they requested as part of the impeachment probe launched in September and ordering members of his administration not to testify.

They say Trump must be removed from office to protect national security and preserve the country’s system of government.

THE EVIDENCE

At the heart of the impeachment case is a transcript of a July 25 telephone call in which Trump pressed Ukraine’s new president, Volodymyr Zelenskiy, to work with U.S. Attorney General William Barr and Trump’s personal attorney, Rudy Giuliani, to launch a corruption investigation into former Vice President Joe Biden, a top contender for the Democratic presidential nomination in 2020.

Trump also asked Zelenskiy to look into a debunked theory that Ukraine, not Russia, interfered in the 2016 presidential election.

The allegations by Trump allies against Biden - that he used his position to force the removal of a Ukrainian prosecutor in order to stop an investigation of an energy company of which his son Hunter was a director - have been discredited. Neither Trump nor his allies have provided evidence to support them, and Biden has denied them.

Current and former U.S. government officials testified during the inquiry that Trump directed them to work with Giuliani, even though the former New York mayor had no official government position.

Some witnesses said they believed that Trump withheld nearly $400 million in security aid, as well as the prospect of a high-profile White House meeting, to pressure Zelenskiy to announce the investigations. Trump’s acting chief of staff, Mick Mulvaney, also told reporters that the White House withheld the money.

The U.S. ambassador to the European Union, Gordon Sondland, provided some of the most damaging testimony here before the House Intelligence Committee. He said he spoke directly with Trump about the effort to pressure Ukraine and said other top administration officials were involved. He testified that Ukrainian officials understood they would have to announce the investigations in order to receive the withheld security aid.

Trump ultimately released the money after news of the delay became public. He has yet to invite Zelenskiy to the White House.

OTHER EVIDENCE

Other evidence has emerged since House Democrats wrapped up their inquiry and voted to impeach Trump in December.

Internal government communications released through open-records requests show White House officials told the Pentagon that Trump had directed them to hold back the aid to Ukraine.

The Government Accountability Office, a non-partisan congressional watchdog, said Trump broke the law by refusing to spend money that had been approved by Congress.

One of Giuliani’s former associates, Florida businessman Lev Parnas, last week provided phone messages and other evidence to the Intelligence Committee that detailed efforts by him and Giuliani to pressure Kiev.

REPUBLICANS

Trump says he has done nothing wrong, and his Republican allies in the House agree with him - not a single one voted for either of the two articles of impeachment.

Trump's legal team argues here that neither of the charges amounts to an impeachable offense, or even a crime, because the president has the right to decide how to conduct foreign policy and what materials to share with Congress.

They say Trump was within his rights to ask Zelenskiy to investigate the Bidens as part of a broad anti-corruption drive.

They also say Trump has the right to resist congressional demands for information.

Republican lawmakers say the Democrats’ case for impeachment amounts to hearsay because it relies mostly on the testimony of mid-level officials who did not deal directly with Trump.

They say Democrats should have gone to court to force the testimony of senior officials who were directly involved in the matter after Trump ordered them not to cooperate - a process that could have taken months.

One official who could provide an eyewitness account, former national security adviser John Bolton, refused to participate in the House inquiry but said he is willing to testify in the Senate trial. It is unclear whether Republicans, who control the chamber, will agree to let him do so.

Republicans also say no actual exchange of favors took place because Zelenskiy ultimately did receive the delayed aid - as well as a meeting with Trump, although that was on the sidelines of the U.N. General Assembly and not at the White House - without agreeing to the investigations Trump wanted.

They argue that Democrats are subverting the will of voters who elected Trump president in 2016 because they do not like him or his policies, and turning the impeachment process into a partisan tool.