Uhm. Nixon? The first article of impeachment was obstruction of justice.
On June 17, 1972, and prior thereto, agents of the Committee for the Re-election of the President committed unlawful entry of the headquarters of the Democratic National Committee in Washington, District of Columbia, for the purpose of securing political intelligence. Subsequent thereto, Richard M. Nixon, using the powers of his high office, engaged personally and through his close subordinates and agents, in a course of conduct or plan designed to delay, impede, and obstruct the investigation of such illegal entry; to cover up, conceal and protect those responsible; and to conceal the existence and scope of other unlawful covert activities.
Trump hasn't obstructed, but it was a DUMB tweet to sloppily lump together the lie to Pence with the lie to the FBI.
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Have you forgotten that no article of impeachment of Nixon was ever passed by congress?
Nixon left office voluntarily when he figured out that they would just give him the JFK treatment.
Presidents cannot obstruct their own justice.
Every decision is ultimately the president’s to make unless he delegates it to his staff.
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The president has the Constitutional power to stop any investigation that is being done from within its own branch, for example an FBI investigation of Watergate.
Now if Nixon interfered with a Congressional investigation, that would be obstruction by a president.