WASHINGTON (AP) - American history is happening in the United States House of Representatives.

Democrats are driving President Donald Trump to the brink of impeachment on Wednesday (Dec 18) as the House takes up charges that Mr Trump abused his power and obstructed Congress in pressuring Ukraine to investigate political rivals and refusing to cooperate with the ensuing congressional probe.

The nation's 45th president is on track to become only the third commander in chief to be impeached.

But first, watch for a day-long showdown that has been boiling for years between Republicans loyal to Mr Trump and Democrats who say his conduct towards Ukraine makes him unfit for office.

Look, too, for legacy moments for Washington's political veterans on the eve of the 2020 election year.

What to watch for during a historic day on Capitol Hill that is expected to end with a final vote Wednesday evening:

SPOILER ALERT

Trump is heading for impeachment. When the House opens debate, the outcome will have been known for some time.

A tally compiled by The Associated Press found that a majority of House members have said they will vote to approve the charges and send them to the Senate for a trial next month.

The GOP-led Senate is not expected to convict and remove Mr Trump from office.

PARTISANS, MOSTLY

Expect most Democrats to vote for impeachment and all Republicans to vote against it.

But there are exceptions.

One freshman Democrat, Mr Jeff Van Drew of New Jersey, has indicated he will oppose impeachment, then switch parties to become a Republican. Earlier this year, Michigan conservative Representative Justin Amash left the GOP when he favoured impeachment. He is expected to vote yes to impeach.

One new Democratic congressman, Mr Jared Golden of Maine, said he would vote to impeach on abuse of power but not obstruction.

AMERICANS

Expect the House to take the nation's us-versus-them political culture out for a spin before a global audience. But it's not clear the proceedings are changing many minds.

Mr Trump's approval ratings have held steady since a whistle-blower report and a partial transcript revealed he had pressured Ukraine's President to investigate Democrats.

Wide shares of Democrats both disapprove of the President and support impeachment, while wide shares of Republicans approve of Mr Trump and want him to remain in office.

New polls from The Washington Post/ABC News and CNN find support for impeachment and removal remains at about half of Americans.

'PROFOUND DISGRACE'

Impeachment will subject Mr Trump to what former presidents Gerald Ford and Jimmy Carter called a "profound disgrace" that stains a president's legacy forever. Only two presidents have been impeached: Mr Andrew Johnson in 1868 and Mr Bill Clinton in 1998.

Mr Richard Nixon chose to resign instead.

Mr Trump has mocked the articles of impeachment as weak.

TRUMP'S DAY

Mr Trump is heading to Michigan, the Democratic state he flipped in 2016. Vice-President Mike Pence is on a bus tour across the state and expected to join him at the rally in Battle Creek.

On the eve of the floor action, Mr Trump gave a nearly six-page preview of his approach in a rambling letter to House Speaker Nancy Pelosi in which he cast himself as a victim and accused the Democrats of smarting over their election losses.

"You are the ones bringing pain and suffering to our Republic for your own selfish, personal political and partisan gain," he wrote.

PELOSI

It's legacy time for Ms Pelosi, too.

In her second turn as speaker, the House veteran of more than three decades says relentlessly that impeachment is a sombre time, not one for celebration. She says the Clinton impeachment proceedings were too divisive for the county and resisted opening impeachment proceedings against president George W. Bush after the Iraq War.

She refused to launch impeachment proceedings against Mr Trump over his obstruction of the Russia investigation, saying she'd be against doing so unless there was bipartisan sentiment for it.

That still doesn't exist for the impeachment articles now headed for passage. But Ms Pelosi has said Mr Trump's conduct toward Ukraine, while holding up military aid to that country, left her with no choice.

"Very sadly, the facts have made clear that the President abused his power for his own personal, political benefit and that he obstructed Congress," she wrote to colleagues. "In America, no one is above the law."

NEXT STOP: SENATE

After votes on each of the two amendments, the House is expected to authorise Ms Pelosi to name a team of prosecutors for the Senate trial. Lobbying for the jobs is well under way. The wagering begins with the two lead House impeachment chairmen, Mr Adam Schiff of Intelligence and Mr Jerrold Nadler of Judiciary, playing roles.

The trial is expected to begin in January, with Chief Justice John Roberts presiding.

Senate Majority Leader Mitch McConnell and Minority Leader Chuck Schumer have begun negotiating, and sniping, over how the trial will be conducted.

Mr McConnell set the partisan tone by declaring that he's "not an impartial juror".