(Adds comment from Representative David Cicilline)
By Sarah N. Lynch, David Morgan and Steve Holland
WASHINGTON, May 20 (Reuters) - U.S. President Donald Trump on Monday told former White House counsel Don McGahn to defy a subpoena to testify before Congress about the Russia investigation, deepening his fight with Democratic lawmakers.
McGahn would "respect the President's instruction" and not testify, his lawyer said later, in a letter sent to the head of the House of Representatives Judiciary Committee.
The committee is investigating whether Trump illegally obstructed the probe into Russian interference in the 2016 presidential election. It wants to quiz McGahn after he figured prominently in a report by Special Counsel Robert Mueller about the Russia probe and whether Trump committed obstruction of justice.
But in a letter to the committee, White House Counsel Pat Cipollone said McGahn should not appear due to both "constitutional immunity" and "in order to protect the prerogatives of the Office of the Presidency."
The committee's Democratic chairman, Jerrold Nadler, responded by saying Trump was trying to block damaging testimony about his obstruction of justice.
"The President acted again and again - perhaps criminally - to protect himself from federal law enforcement. Don McGahn personally witnessed the most egregious of these acts. President Trump knows this. He clearly does not want the American people to hear firsthand about his alleged misconduct," Nadler said in a statement.
He said his committee would meet on Tuesday morning and expected McGahn to show up and testify.
Another Democrat on the committee, David Cicilline, warned that if McGahn stays away on Tuesday, lawmakers might need to push ahead with an impeachment inquiry against Trump in order to compel his administration to produce witnesses and documents.
"There are a number of colleagues who have suggested that if Mr. McGahn doesn't appear tomorrow and answer questions that it's an appropriate time to open an impeachment inquiry," Cicilline told reporters.
He said an inquiry would begin the process to determine whether or not articles of impeachment should be voted on.
In his report, Mueller cited McGahn as saying that Trump called him several times in June 2017 to tell him to direct the Justice Department to remove Mueller because of conflicts of interest.
McGahn did not carry out Trump's order. Later, when news articles about the incident surfaced, McGahn told Mueller's investigators that Trump tried to get him to dispute the accuracy of the reports. McGahn again refused.