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The top Democrat in the US Congress, Nancy Pelosi, has written to President Donald Trump inviting him to give his State of the Union speech on 4 February amid the partisan impeachment battle.

The annual speech would come during or soon after his trial in the Senate, where he is likely to be acquitted.

He was formally impeached by the House of Representatives on Wednesday.

House Speaker Pelosi told Associated Press he would be "impeached forever, no matter what the Senate does".

The impeachment vote two days ago in the Democrat-led House of Representatives split almost totally along party lines.

Democrats and Republicans have yet to agree on when the upcoming Senate trial will take place.

What did the letter say?

The letter sent by Mrs Pelosi on Friday begins with a pointed reminder to Mr Trump of the "separation of powers" enshrined in the US Constitution.

The three branches - the judicial, executive, and legislative - she said are "co-equal branches acting as checks on each other".

"In the spirit of respecting our Constitution, I invite you to deliver your State of the Union Address," she wrote, adding: "Thank you for your attention to this matter".

White House spokesman Hogan Gidley said that Mr Trump had accepted the invitation to speak.

A top Republican in congress, Texas Senator John Cornyn, reacted to the invitation with the hashtag "#somanymixedsignals".

"Guess she expects him to still be in office then," he tweeted.

Mr Trump faces two charges, or articles of impeachment: abusing his power by requesting that Ukraine investigate his political rival so as to gain political advantage; and obstructing Congress by refusing to co-operate with the impeachment investigation.

The second charge came after Mr Trump refused to allow White House officials to testify and withheld documents. Democrats charge that this breached constitutional protocol which requires Congress to provide oversight of the White House executive branch.

What has Mr Trump said?

The Democrats and Republicans are wrangling over how the Senate trial should be conducted, casting doubt over when it will start.

This prompted a series of tweets from the president on Thursday accusing the Democrats of not wanting to start the trial because their "case is so bad" and demanding it begin immediately.

He tweeted: "So after the Democrats gave me no Due Process in the House, no lawyers, no witnesses, no nothing, they now want to tell the Senate how to run their trial. Actually, they have zero proof of anything, they will never even show up. They want out. I want an immediate trial!"

Media playback is unsupported on your device Media caption "Cheapened the process of impeachment" - Trump reacts at a rally

The president said the Democrats did not want Congressman Adam Schiff, who led the impeachment process, the Bidens and a CIA whistleblower who sparked the inquiry to testify.

The Democrats have argued that it is Mr Trump's Republicans who are balking at the appearance of witnesses. The House did also invite the president to testify before its investigators but he declined to do so.

What else is happening in early February?

The Tuesday night primetime speech will come only one day after Democrats hold their first-in-the nation nominating vote for the 2020 presidential election.

The Iowa caucus will be the first time that voters pick their preferred candidate to take on Mr Trump in November.

The speech will come two days after another primetime event. Super Bowl Sunday - the US football championship - is the most-watched television event in the US each year.

Later that week, on Friday, Democrats will hold another primary debate in New Hampshire - another early voting state.

A Senate impeachment trial may have already begun, or even be over, as Mr Trump speaks, but the actual date has yet to be confirmed amid the impasse among lawmakers.

Media playback is unsupported on your device Media caption Highlights of the 2018 speech: An eye-roll, a chant and an unexpected ovation

Why is there deadlock over the start of the trial?

To start the next stage, the Democrat-controlled House must send the articles of impeachment to the Senate.

But House Speaker Nancy Pelosi is refusing to do so until the rules of the Senate trial are acceptable to the Democrats.

The Senate's Republican leader, Mitch McConnell, will determine the terms of the trial and the Democrats want him to provide details on which witnesses and what testimony will be allowed.

He has so far refused to do so. "We remain at an impasse," he said, after a brief meeting with the Democrats' Senate minority leader, Chuck Schumer.

Mr McConnell leads the majority in the Senate, with 53 Republicans in the 100-seat chamber.

Media playback is unsupported on your device Media caption Mitch McConnell, who will decide the trial's terms, condemns the "unfair impeachment inquiry"

Mr McConnell has called the impeachment process the "most rushed, least thorough and most unfair" in history.

The Democrats hope the delay will both move public opinion in favour of a fuller trial and deny Mr Trump - only the third US president to be impeached - a swift acquittal.

The Democrats want at least four current and former White House aides with knowledge of the Ukraine affair to testify.

They say the trial has to be fair, with senators acting as impartial jurors, and that Mr McConnell's comments show he has no plans to do this. He earlier said Republican senators would act in "total co-ordination" with the president's team.

What is the president accused of?

He is accused of having withheld $400m (£307m) of military aid to Ukraine already allocated by Congress, and a White House meeting for Ukraine's new president, until Ukraine looked into potentially damaging material on Joe and Hunter Biden.

Hunter worked for a Ukrainian company when Joe Biden was US vice-president.

The Democrats say this amounts to an abuse of presidential power, using the office for personal political gain and to the detriment of national security.

Mr Trump is also accused of obstructing Congress by refusing to co-operate with the congressional inquiry.

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