Washington DC - The unprecedented second impeachment trial of former President Donald Trump begins in the US Senate on Tuesday.

The US has only had four presidential impeachments in its history, two of which were of Trump and took place over the last year. No other president or federal official has been impeached twice.

The US House of Representatives has charged Trump with incitement of an insurrection for his involvement in a deadly riot at the US Capitol on January 6.

The events took place after Trump refused to admit that he lost the November 2020 presidential election to current President Joe Biden (78), and instead promoted unsubstantiated claims of voter fraud.

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Many of Trump's supporters believed the conspiracy theories that argued the vote was stolen.

During a rally in Washington on January 6, scheduled as a joint session of Congress met to certify the results of the election, Trump (74) urged his supporters to go to the Capitol building to protest.

Shortly thereafter, thousands of angry Trump supporters stormed the US Capitol, breaking windows, stealing documents and sending US lawmakers into hiding. Five people died, including one policeman.

Among those who stormed the Capitol were members of white supremacist and self-styled militia groups.

Democrats in Congress argued that Trump should be immediately impeached for inciting the violence. The proceedings, however, were delayed until after Biden's inauguration.

In a brief filed on Monday, Trump's lawyers asked the Senate to dismiss the case against their client.

Their defense is largely based on the argument that a president cannot be impeached after leaving office. Many constitutional law scholars have dismissed this argument, however.

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Democratic lawmakers argue that it is imperative for Trump to be convicted in order to safeguard US democracy.

Trump's conduct "endangered the life of every single member of Congress" and "jeopardized the peaceful transition of power and line of succession," impeachment managers wrote in a pre-trial brief.

Trump was acquitted by the Republican-controlled Senate during his first impeachment, which took place in 2019 after Trump ordered a freeze on 391 million dollars of security aid to Ukraine that Congress had already approved.

Democrats in the House began the impeachment process after a whistle-blower complaint revealed that Trump attempted to use the aid as leverage to extract political favors from Ukraine's leadership.

During a call with Ukraine's president, Trump hinted that he would release the assistance if Ukraine opened an investigation into Biden.

The Senate must vote by a two-thirds majority to convict in an impeachment, and Republican lawmakers were unwilling to vote against a popular member of their party.

It is likely that the same thing will happen in the second impeachment trial.

Democrats now control the Senate by the slimmest possible majority, but they do not appear to have the support of the necessary number of Republican lawmakers to convict Trump.

At least 17 Republican Senators would need to vote against Trump in the 100-member chamber.

Republican Senators are wary of alienating Trump's highly mobilized political base and wish to avoid censuring him publicly by voting to convict. Still, most have stopped short of defending his actions.

Trump's first impeachment trial lasted nearly three weeks. But this trial is expected to proceed much quicker, with a final vote perhaps coming as soon as early next week.

The fast tempo is partly a result of the case against Trump being simpler this time around – a bounty of video evidence and witness accounts are at hand. Democrats are also keen not to bury Biden's legislative agenda in Congress with the trial.

Like in the first impeachment, Trump has refused to appear before the Senate to testify.