"the senate shall have the sole power to try all impeachments. when sitting for that purpose, they shall be on oath or affirmation. when the president of the united states is tried, the chief justice shall preside: and no person shall be convicted without the concurrence of two thirds of the members present." —article i, section 3 because of the principle of the constitution states that congress can impeach a sitting president if he or she behaves in an unlawful manner.