Trump isn't the first president to be subpoenaed by Congress: Here are the others

The House committee investigating the Jan. 6, 2021, riot on Capitol Hill unanimously voted on Thursday to subpoena former President Donald Trump, escalating the panel's efforts to dig into the impetus and impacts of the insurrection.

While there have been several judicial subpoenas for presidents before, congressional subpoenas for sitting or former White House occupants are rare -- but not unheard of. (Trump did not immediately say Thursday whether he would comply or fight once the subpoena was issued.)

Separately, numerous presidents have voluntarily testified before Congress, but no sitting president has been forced to appear.

Here are the other presidents, sitting or former, who have been subpoenaed by Congress.

John Tyler and John Quincy Adams

Congress subpoenaed the two presidents after they had already left office. The orders for testimony in 1848 stemmed from accusations by the House Committee on Foreign Affairs that then-Secretary of State Daniel Webster misused money for a secret "contingent fund" used by presidents for clandestine intelligence operations.

Spending from the fund required presidential signatures, which led Congress to seek the testimonies of Adams and Tyler. In office at the time, President James Polk refused to provide detailed information to Congress out of deference to the prior administrations.

The two former presidents still cooperated, with two select investigative committees questioning Tyler while Adams provided a sworn deposition to one of them.

Harry Truman

The infamous House Committee on Un-American Activities, which reached its height during the Cold War-era Red Scare, subpoenaed former President Harry Truman to get information on his "Loyalty Program," which was intended to root out anyone loyal to the Soviet Union.

The subpoena was issued after he left office and centered around Truman's appointment of an assistant treasury secretary who was rumored to have communist ties.

Truman said in a letter that he would not accommodate the subpoena in spite of his "personal willingness" to cooperate, citing the Constitution's separation of powers.

"If your intention is to inquire into any acts as a private individual either before or after my Presidency and unrelated to any acts as President, I shall be happy to appear," he wrote.

Richard Nixon

Nixon was actually subpoenaed twice by Congress, both in relation to the Watergate scandal.

The Senate panel probing the Watergate break-in subpoenaed Nixon in 1974 for tapes and records on more than one occasion, though he declined, and the court ultimately rejected an effort by the committee to enforce one of the subpoenas.

However, the House Judiciary Committee, which was also investigating the burglary that ended up costing Nixon his career, subpoenaed documents that Nixon did end up handing over -- though the committee didn't think he had turned over all he had.