A historic, unprecedented event is about to unfold in Washington. Senators on Tuesday begin a second impeachment trial of former President Donald Trump. The House passed an article of impeachment on Jan 13, charging him with "incitement of insurrection" in connection with the Capitol riot Jan. 6, and now the process turns to the Senate. Democrats from the House will present evidence -- mainly his words at a "Stop the Steal" rally right before the riot, when he urged thousands of attendees to go to the Capitol. Defense lawyers will appear on Trump's behalf, and last week rejected a Democratic request for him to answer questions. If Democrats want to push it, impeachment trial rules say the Senate can "compel the attendance of witnesses" – meaning they could issue a legal document called a subpoena (pronounced sub-PEEN-ah) that requires his testimony, though that would risk a separate court fight delaying the trial.

The longest-serving Senate Democrat, Patrick Leahy of Vermont, will preside over the impeachment trial. Conviction requires at least 67 of the 100 senators. Republican leaders tried unsuccessfully last week to prevent the trial by claiming it's unconstitutional to put an ex-president on trial this way. Democrats say it's necessary to hold him accountable and, potentially, to keep him from holding any federal office again -- as senators could decide by majority vote if he's convicted. (The chamber has 50 Democrats and 50 Republicans, with Vice President Kamala Harris able to break ties.) The House's impeachment language says Trump "warrants . . . disqualification to hold and enjoy any office of honor, trust or profit under the United States."

The previous three presidential impeachment – of Andrew Johnson in 1868, Bill Clinton in 1998 and Trump himself in early 2020 – were conducted while the accused were still in office. A public policy professor at the University of Washington, Michael Blake, says this trial is an appropriate way to define boundaries of presidential behavior. "It will set the moral limits of the presidency – and, thereby, send a message to future presidents," he explains.

Among the public, a recent Associated Press poll shows that nearly two-thirds of Americans believe that Trump bears at least a moderate amount of responsibility for the breach of the U.S. Capitol -- including half who say he bears a great deal or quite a bit. Just over a third say he has little to no responsibility. Forty-seven percent of the 1,055 adults surveyed Jan. 28-Feb. 1 believe senators should convict Trump. Another 40 percent say he should not be convicted and 12 percent are unsure.

Democrat says: "The facts are very clear that you had the president of the United States basically inciting an insurrection and … a violent attack on the United States Capitol. . . . The former president now needs to be held accountable for that behavior." – Sen. Gary Peters, D-Mich.

Republican says: "I think it's kind of ridiculous. Are we going to start impeaching all the past presidents we don’t like?" -- Bill Stokes, 67-year-old Trump voter in Casper, Wyo.

Columnist says: "That Trump won't testify in his own defense will only further underscore how indefensible all this really is." – Greg Sargent, The Washington Post