WASHINGTON (AFP) -- Donald Trump's second impeachment trial began Tuesday with harrowing video footage of his supporters' assault on Congress, but Republican senators made clear how difficult it will be to win a conviction.

Senators voted 56-44 in favor of the constitutionality of the historic trial, rejecting a bid by Trump's lawyers to throw it out on grounds that a former president cannot be tried by lawmakers.

The vote, held before the main part of the trial was to get underway Wednesday, saw six Republicans join all 50 Democrats in the evenly divided Senate.

Despite this modest show of bipartisanship, the result highlighted the nearly impossible task of getting the two thirds majority -- requiring 17 Republicans to join the Democrats -- that would be needed to convict Trump of inciting insurrection.

Earlier, both sides presented their opening cases, with Democrats arguing that Trump broke his oath in a naked bid to retain power after losing the November election to Joe Biden.

Refusing to accept his defeat, Trump spread lies about vote rigging and repeatedly pressured officials, including then vice president Mike Pence, to try and stop the transfer of power.

Finally, on January 6, Trump told enraged Republican supporters near the White House to "fight like hell." The crowd, chanting "stop the steal," then attacked Congress, where Pence and lawmakers were in the process of certifying Biden's victory.

"If Congress were to just stand completely aside in the face of such an extraordinary crime against the Republic, it would invite future presidents to use their power without any fear of accountability," Democratic impeachment manager Joe Neguse said.

Video from the January 6 mayhem played back inside the ornate Senate packed the biggest punch.

Senators -- who witnessed the events first hand when they had to be rushed to safety that day -- watched raw footage of Trump's speech and the crowd's ensuing assault on the Capitol.

The video montage showed the mob chanting pro-Trump slogans as it smashed through the doors, swarmed over police, and managed for the first time in history to disrupt the congressional vote certifying the election.

"If that's not an impeachable offense, then there is no such thing," lead impeachment manager Jamie Raskin said.

Fighting tears, Raskin recounted how he and his family -- who were visiting to watch the certification -- had been trapped, listening to "the sound of pounding on the door like a battering ram, the most haunting sound I have ever heard."

"This cannot be the future of America," he implored senators.

Trump laywer David Schoen, however, said the Senate had no jurisdiction to try Trump once he had left office and warned that the impeachment threatened to "tear this country apart."

It will leave the United States "far more divided and our standing around the world will be badly broken," he argued.

Trump sets record

Trump is the first president ever to face two impeachment trials -- he was already acquitted in 2020 of abuse of power -- as well as the first in history to be tried after leaving the White House.

His team is basing its case largely on the procedural argument that a former president cannot be tried, calling the Senate trial "absurd."

They also argue that whatever Trump said during his January 6 rally is protected by the constitutional right to free speech and did not amount to ordering the assault on Congress.

The trial is clearly uncomfortable for many Republican senators, who, like their Democratic colleagues, had to flee to safety during the violence. Reminders of the mayhem are everywhere around them, with thousands of National Guard troops still deployed around the newly fortified Capitol building.

Despite this, a second acquittal is all but certain for Trump, who is holed up in his luxury Mar-a-Lago resort in Florida.

Amped up on four years of Trump's populist claims to be fighting for ordinary people against the elites, huge numbers of Republican voters continue to support the ex-president, pushing their party ever further to the right.

Polls show that a small majority of the country overall believes Trump deserves conviction. An Ipsos/ABC News poll found 56 percent back this, while a Gallup poll found 52 percent support.

Among Republicans, however, an overwhelming majority opposes convicting Trump.

Biden above the fray

Biden, who succeeded Trump on January 20, is attempting to stay above the fray.

Daily, the White House is sending a message that the Democrat is focused instead on the fragile economy and the desperate effort to vaccinate Americans against the still out-of-control Covid-19 pandemic.

When asked on Tuesday about the trial, Biden offered a short few words.

"Look, I told you before... I have a job," he said. "The Senate has their job and they are about to begin it and I am sure they are going to conduct themselves well.

"That's all I am going to say about impeachment."

If Trump were convicted, the Senate would then hold a simple-majority vote on barring him from future public office.

But even if the impeachment trial ends in acquittal, calls to punish Trump for his behavior will likely continue, including possibly a push for a bipartisan vote of censure.