The Democrat-dominated Judiciary Committee in the House of Representatives has issued a subpoena compelling McGhan to testify.
But the White House said that special counsel Robert Mueller's sprawling probe into Trump's links to Russia and alleged attempts to get in the way of the investigation had cleared the president, meaning there was no need for more digging.
Mueller submits report on bombshell Trump-Russia probe
McGhan himself testified before Mueller's staff for some 30 hours.
"The Democrats do not like the conclusion of the Mueller investigation... and want a wasteful and unnecessary do-over," Sanders said in a statement.
"The Department of Justice has provided a legal opinion stating that, based on long-standing, bipartisan, and Constitutional precedent, the former Counsel to the President cannot be forced to give such testimony, and Mr McGahn has been directed to act accordingly," she said.
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