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The Senate vote of 57-43 on Saturday fell short of the two-thirds majority (67 votes) the 100-seat chamber needed to convict Trump on a charge of incitement of insurrection after a five-day trial in the same building ransacked by his followers on January 6 shortly after they heard him deliver an incendiary speech.
Trump said the trial had been another phase of the "greatest witch hunt in the history of our country."
In the vote, seven of the 50 Senate Republicans joined the chamber's unified Democrats in favouring conviction.
Trump left office on January 20, so impeachment could not be used to remove him from power. But Democrats had hoped to secure a conviction to hold him responsible for a siege that left five people including a police officer dead and to set the stage for a vote to bar him from ever serving in public office again. Given the chance to hold office in the future, they argued, Trump would not hesitate to encourage political violence again.
Republicans also saved Trump in the February 5, 2020, vote in his previous impeachment trial, when only one senator from their ranks - Mitt Romney - voted to convict and remove him from office.
TRUMP 'MORALLY RESPONSIBLE' FOR ATTACK
But US Senate Minority Leader Mitch McConnell called Trump "practically and morally responsible" for his supporters' deadly attack on the Capitol, only moments after voting to acquit him on a charge of inciting the melee.
The top Senate Republican explained the unexpected turnabout at the end of the five-day impeachment trial, by declaring it unconstitutional to convict Trump of misconduct now that the former president has left office and become a private citizen.
"There is no question that President Trump is practically and morally responsible for provoking the events of the day," said McConnell, who along with the rest of the Congress and former Vice President Mike Pence fled the mob that descended on the Capitol on January 6.
"The people who stormed this building believed they were acting on the wishes and instructions of their president," McConnell said.
In comments that echoed the prosecution case presented by House managers, McConnell said Trump had orchestrated "an intensifying crescendo of conspiracy theories" and described the former president as "determined to either overturn the voters' decision or else torch our institutions on the way out."
Seven of McConnell's Republicans voted to convict. He suggested that Trump could yet face criminal prosecution for his acts.
"President Trump is still liable for everything he did while he was in office as an ordinary citizen," McConnell said. "He didn't get away with anything. Yet."
The drama on the Senate floor unfolded against a backdrop of gaping divisions in a pandemic-weary United States along political, racial, socioeconomic and regional lines. The trial provided more partisan warfare even as Democratic President Joe Biden, who took office on January 20 after defeating Trump at the ballot box on November 3, called for healing and unity after his predecessor's four turbulent years in power and a caustic election campaign.
Trump continues to hold a grip on his party with a right-wing populist appeal and "America First" message. The wealthy businessman-turned-politician has considered running for president again in 2024.
The 74-year-old is only the third president ever to be impeached by the House of Representatives - a step akin to a criminal indictment - as well as the first to be impeached twice and the first to face an impeachment trial after leaving office. But the Senate still has never convicted an impeached president.
Democrats forged ahead with impeachment despite knowing it could overshadow critical early weeks of Biden's presidency.
The House approved the single article of impeachment against Trump on Jan. 13, with 10 Republicans joining the chamber's Democratic majority. That vote came a week after the pro-Trump mob stormed the neoclassical domed Capitol, interrupted the formal congressional certification of Biden's victory, clashed with an overwhelmed police force, invaded the hallowed House and Senate chambers, and sent lawmakers into hiding for their own safety.
'FIGHT LIKE HELL'
Shortly before the rampage, Trump urged his followers to march on the Capitol, repeated his false claims that the election was stolen from him through widespread voting fraud, and told them that "if you don't fight like hell, you're not going to have a country anymore."
During the trial, nine House lawmakers serving as trial managers, or prosecutors, urged senators to convict Trump to hold him accountable for a crime against American democracy and to prevent a repeat in the future.
They played searing video of rioters swarming inside the Capitol and making violent threats toward politicians including House Speaker Nancy Pelosi and then-Vice President Mike Pence. The House managers said Trump summoned the mob to Washington, gave the crowd its marching orders and then did nothing to stop the ensuing violence.
The defence lawyers accused Democrats not only of trying to silence Trump as a political opponent they feared facing in the future but of attempting to criminalise political speech with which they disagreed and aiming to cancel the voices of the tens of millions of voters who backed him.
Trump's lawyers argued the trial was unconstitutional because he had already left office and that his remarks were protected by the constitutional right to free speech. The words Trump used, they argued, were no different than those regularly employed by Democrats.
In his previous impeachment trial, the Senate voted to acquit Trump on two charges - abuse of power and obstruction of Congress. That impeachment arose from Trump's 2019 pressure on Ukraine to investigate Biden as he sought foreign aid to sully a domestic political rival.
A common theme in the charges at the heart of the two impeachments was Trump's abandonment of accepted democratic norms to advance his own political interests.
The US Constitution sets out impeachment as the instrument with which the Congress can remove and bar from future office presidents who commit "treason, bribery or other high crimes and misdemeanours."
Impeachment, once a rare occurrence, has become more commonplace during America's era of poisonous political polarization in recent decades. In the 209 years after the first US president, George Washington, took office in 1789, there was only one impeachment.
Since 1998, there have been three, including Trump's two. Andrew Johnson was impeached and acquitted in 1868 in the aftermath of the American Civil War and Bill Clinton was impeached in 1998 and acquitted in 1999 of charges stemming from a sex scandal.
Richard Nixon resigned in 1974 rather than face impeachment over the Watergate scandal.
Trump's acquittal does not end the possibility of other congressional action against him such as a censure motion. Republicans seemed dead set against an idea floated by Democrats of invoking the Constitution's 14th Amendment provision barring from public office anyone who has "engaged in insurrection or rebellion" against the government.
The impeachment proceedings also can be viewed in the context of a battle for the future of the Republican Party. Some Republicans - mostly moderates and establishment figures - have voiced alarm at the direction Trump has taken their party.
Detractors have accused Trump - who had never before held public office - of undermining the institutions of democracy, encouraging a cult of personality and pursuing policies built around "white grievance" in a nation with a growing non-white population.