By David Morgan and Richard Cowan
WASHINGTON (Reuters) - Donald Trump's historic impeachment trial on a charge of inciting last month's deadly storming of the U.S. Capitol began on Tuesday, making the Republican the first former U.S. president to be tried in the Senate.
House of Representatives Democrats serving as prosecutors opened their case by showing video of Trump supporters violently overwhelming police at the Capitol in the Jan. 6 attack after he had encouraged people in a speech to "fight like hell" to overcome his Nov. 3 election defeat.
The video showed Trump backers throwing down barriers, hitting police officers and at one point telling one: "We outnumber you a million to one out here."
Convicting Trump would require a two-thirds vote in the 100-member chamber, meaning that at least 17 Republicans would need to join the Senate's 48 Democrats and two independents in voting against Trump. That is a tall order.
In the Capitol assault, the mob attacked police, sent lawmakers scrambling for safety and interrupted the formal congressional certification of President Joe Biden's victory after Trump had spent two months challenging the election results. Five people died, including a police officer.
Senate Majority Leader Chuck Schumer, a Democrat, called the accusation against Trump: "The gravest charges ever brought against a president of the United States in American history."
Trump, a Republican, was impeached by the Democratic-led House on Jan. 13 on a charge of inciting an insurrection, becoming the only president to have been impeached twice and the only former president to face a Senate trial.
Before the attack, he claimed falsely that widespread voting fraud lost him the election to Biden.
Democratic congressman Jamie Raskin, who is leading the prosecution, said presidents would create a "January exception" that allowed them to break the law with impunity in the last month of their term if senators did not convict Trump.
"The January exception is an invitation to our founders' worst nightmare. ... We risk allowing Jan. 6 to become our future," he told the trial, in which senators serve as jurors.
Defense lawyers plan to argue on Tuesday that only a sitting president can face an impeachment trial.
But a majority of legal experts say it is constitutional to have the trial after an official has left office, said Michigan State University law professor Brian Kalt, a leading impeachment scholar.
Senate Democrats are expected to prevail in Tuesday's vote on the constitutionality of the trial. A Republican effort to block the trial on those grounds was defeated 55-45 last month.
The trial is being held with extraordinary security around the Capitol in the wake of the siege including armed security forces and a perimeter of fencing and razor wire.
The trial could provide clues on the direction of the Republican Party following Trump's tumultuous four-year presidency. Sharp divisions have emerged between Trump loyalists and those hoping to move the party in a new direction. Meanwhile, Democrats are concerned the trial could impede Biden's ability to swiftly advance an ambitious legislative agenda.
Trump's defense has also argued he was exercising his right to free speech under the Constitution's First Amendment when he addressed supporters at a rally in Washington on the day of the Capitol attack and urged them to "fight" against the election result.
In a pre-trial document released on Tuesday, the House managers described that approach as "legally frivolous."
"Accepting President Trump's argument would mean that Congress could not impeach a President who burned an American flag on national television, or who spoke at a Ku Klux Klan rally in a white hood, or who wore a swastika while leading a march through a Jewish neighborhood - all of which is expression protected by the First Amendment but would obviously be grounds for impeachment," the Democrats said.
On Wednesday, the prosecution and defense are due to turn to the merits of the charge. They will have 32 hours evenly divided over no more than four days to present their cases. The proceedings could be extended further, with senators having time to question both sides.
One year ago, the then Republican-controlled Senate acquitted Trump on charges of obstructing Congress and abuse of power related to his pressure on Ukraine to launch an investigation into Biden and his son Hunter in 2019.
(Reporting by Will Dunham)