Open this photo in gallery U.S. Speaker of the House Nancy Pelosi, seen here on Jan. 9, 2020, has delayed delivery of the charges for weeks to compel Senate Majority Leader Mitch McConnell to agree to include new witness testimony and evidence about Trump’s pressure on Ukraine to probe former Vice President Joe Biden. Tom Brenner/Reuters

U.S. President Donald Trump and Speaker Nancy Pelosi faced off Sunday ahead of his impeachment trial, as she said senators will “pay a price” for blocking new witnesses and he quickly retorted that she and House Intelligence Committee Chairman Adam Schiff should both testify.

The House plans to vote this week to transmit the articles of impeachment to the Senate for the historic trial on charges of abuse of power and obstruction of Congress over Trump’s actions toward Ukraine. It will be only the third presidential impeachment trial in American history and could start this week.

Mr. Trump and Ms. Pelosi, the two most powerful party leaders in the nation, communicated as often happens in this presidency – with the president responding on Twitter to a television interview.

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“It’s about a fair trial,” Ms. Pelosi told ABC’s This Week. “We’ve done our job. We have defended the Constitution of the United States. We would hope the Senate would do that as well.”

She warned, “Now the ball is in their court to either do that or pay a price.”

Mr. Trump tweeted right before and after Ms. Pelosi’s appearance, in both instances using derisive nicknames. He said both she and Mr. Schiff should appear in the Senate for testimony.

“He must be a Witness, and so should she!” Mr. Trump tweeted.

The president rebutted Ms. Pelosi’s suggestion that no matter what the Senate does, he will always be impeached. Ms. Pelosi said the House vote last month means Mr. Trump will be “impeached forever” and “for life.”

“Why should I have the stigma of Impeachment attached to my name when I did NOTHING wrong?” Mr. Trump tweeted, calling the House action a “totally partisan Hoax.“’

Voters are divided over impeachment much the way they are split along partisan lines and as the Senate prepares for the landmark trial, both parties are trying to set the terms of the debate over high crimes and misdemeanours.

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Senate Majority Leader Mitch McConnell (R, Kentucky), is seeking a speedy trial to acquit the president and is reluctant to seek more witnesses. The GOP leader has proposed a process similar to the last presidential impeachment trial of Bill Clinton in 1999 that would start the proceedings and then vote later on hearing new testimony.

One leading Republican, Sen. Lindsey Graham of South Carolina, has already predicted that the trial would end ”in a matter of days.”

In a Fox News Channel interview on Saturday, Mr. Graham dismissed Ms. Pelosi’s tactics, saying the delay would have no effect on calling new witnesses or the expected outcome – acquittal by the GOP-controlled Senate.

“The Senate should not reward this behaviour by the House,” said Mr. Graham, chairman of the Senate judiciary committee. “The Senate should end this trial as quickly as possible. That’s what I intend to do. He will be acquitted. I hope and pray every Republican will reject what Nancy Pelosi did, and we’ll pick up a few Democrats.”

Mr. Trump was charged with abuse of power for pressuring the president of Ukraine to investigate Democrats, specifically Mr. Trump’s political rival Joe Biden. Mr. Trump was also charged with obstruction of Congress for trying to block the House investigation.

Mr. Trump was delaying nearly US$400-million in aide as Ukraine battles Russia on its border while he pushed the country’s new president to investigate. Mr. Trump follows a conspiracy theory pushed by his personal lawyer, Rudy Giuliani, about Mr Biden and his son, Hunter Biden, who served on the board of a gas company there while his father was U.S. vice-president. Neither is accused formally of any wrongdoing.

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Some Republicans want to turn the impeachment trial away from the Democrats’ case against Mr. Trump and toward Mr. Giuliani’s theory about Mr. Biden.

GOP Sen. Rick Scott of Florida said Sunday he wants to hear from the Bidens "and find out – get to the bottom of that.“

Mr. McConnell is reluctant to pursue any more witnesses at all, wary of dragging out the Senate trial. He joined some Republicans in backing a proposal for votes to dismiss the charges against Mr. Trump.

But at least one Republican up for re-election, Sen. Susan Collins of Maine, said last week she was in talks with GOP colleagues on a process that would allow them to hear more testimony as Democrats want.

The Democratic-run House is set to vote this week to send over the articles of impeachment after Ms. Pelosi ended a more than three-week delay.

Once the Republican-led Senate receives the charges, the trial is expected to begin swiftly.

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The date is not yet certain and Ms. Pelosi will meet behind closed doors with House Democrats to decide next steps on Tuesday morning ahead of the party’s presidential primary debate that evening, the last before the Iowa caucus Feb. 3.

While some Democrats have grumbled about the delay, Ms. Pelosi and other party leaders defended the strategy, saying it produced new potential evidence and turned public attention on the upcoming trial.

“One of the things that holding on to the articles has succeeded doing is fleshing out McConnell and the president’s desire to make this a cover-up,” Mr. Schiff said on CBS’ Face the Nation.

“If McConnell succeeds in making this trial a trial without witnesses … That’s not a fair trial. That’s a sham,” he said.

Ms. Pelosi said senators need to consider new witnesses, including former national security adviser John Bolton, who has said he would be willing to testify if he receives a subpoena.

Mr. Trump is blocking White House officials from appearing and reiterated last week he does not want his former top security adviser to testify before the Senate.

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Mr. Bolton is a brash figure and his outspoken comments could cut different ways in testimony. House Democrats, who did not issue a subpoena for Bolton last year, did not rule out doing so now.

“It’s certainly something that we are considering,” Mr. Schiff said.

Ms. Pelosi also left open the door to filing more articles of impeachment against Mr. Trump.

“It’s Sunday morning – let’s be optimistic about the future … a future that will not have Donald Trump in the White House, one way or another, 10 months from now we will have an election, if we don’t have him removed sooner,” she said.

Right before Ms. Pelosi was set to appear for the Sunday interview, Mr. Trump tweeted against Ms. Pelosi, calling her a derisive nickname, “Crazy Nancy.”

Asked about Mr. Trump’s tweet, Ms. Pelosi said, “Every knock from him is a boost.”

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