(CNN) The assertion of Donald Trump's lawyer that a president can never be guilty of obstructing justice because he is the country's top law enforcement officer recalls Richard Nixon's remark that "when the president does it, that means it is not illegal," and intensifies debate over whether a sitting president can be indicted.

Whether a president can be criminally charged -- for any offense -- has never been tested in the courts. But presidents have been subject to obstruction-of-justice charges in impeachment proceedings. And there is no question that a president can be removed for, as the US Constitution dictates, any "high crimes and misdemeanors."

Unlike a criminal case heard by a judge or jury, impeachment is a political process that comes down to votes: a majority in the US House of Representatives to impeach and a two-thirds vote of the US Senate to convict. Yet both sets of proceedings can follow the kind of special counsel investigation now underway. Comparisons to the Nixon scandal have been rife recent months. In Watergate, Nixon was not criminally charged but was named as an unindicted co-conspirator and pressured to resign with impeachment charges looming.

The deal between special counsel Robert Mueller and Trump's former national security adviser Michael Flynn ramped up controversy over whether Trump had known Flynn had lied to the FBI, as Flynn pleaded guilty to last Friday, and perhaps tried to interfere with the federal investigation of Flynn. Former FBI Director James Comey, who oversaw the Department of Justice probe related to Russia interference in the 2016 election before being fired by Trump in May, said Trump had asked him to stop pursuing Flynn.

A Trump tweet on December 2, 2017 , suggested Trump knew before that request to Comey that Flynn had lied when he was fired. If so, that could increase the possibility that Trump was trying to impede Comey's pursuit of Flynn's potentially criminal behavior.

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