Donald Trump’s former campaign manager Corey Lewandowski says he will defend the president over allegations of obstruction as he gives evidence to Congress today in the first high-profile impeachment hearing since the Mueller report.

“Excited about the opportunity to remind the American people today there was no collusion no obstruction,” Mr Lewandowski wrote ahead of his appearance at the House judiciary committee. “There were lots of angry Democrats who tried to take down a duly elected President.”

Mr Lewandowski, a political commentator and operative reportedly considering launching a Republican campaign for Senate in his home state of New Hampshire, also used the hashtag #Senate2020 and added: “Tune in.”

Late on Monday, the White House instructed Mr Lewandowski not to discuss conversations he had had with Mr Trump after he became president, including an exchange at the White House that Democrats view as evidence that the president obstructed justice and may justify him being impeached.

Mr Lewandowski is the first impeachment witness to appear before the House judiciary committee since former Special Counsel Robert Mueller testified in July about his probe of Russian election interference and Mr Trump’s alleged efforts to impede the investigation.

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Democrats, who hope to decide whether to recommend Mr Trump’s impeachment to the full House by the end of the year, had intended to grill Mr Lewandowski about an effort by the president to persuade then-Attorney General Jeff Sessions to redirect the Mueller probe away from his 2016 campaign.

The episode is among a number of incidents contained in the special counsel’s 448-page Russia investigation report that Democrats view as evidence Mr Trump obstructed justice.

Mr Mueller made no determination about whether Mr Trump obstructed justice but did not exonerate him of possible wrongdoing.

White House Counsel Pat Cipollone told the committee in a letter on Monday that Mr Lewandowski could not testify about conversations with Mr Trump after he became president or with his senior advisers.

The White House also ordered two other witnesses, former White House aides Rob Porter and Rick Dearborn, not to testify.

Mr Cipollone’s letter said they were “absolutely immune from compelled congressional testimony with respect to matters related to their service as senior advisers to the President”.

The judiciary committee’s chairman, Democrat Jerrold Nadler, has denounced the White House move as a “shocking and dangerous assertion of executive privilege and absolute immunity.”’

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“If he were to prevail in this cover-up while the judiciary committee is considering whether to recommend articles of impeachment, he would upend the separation of powers as envisioned by our founders,” Mr Nadler said in a statement, referring to the president.