Washington: The historic impeachment trial of US president Donald Trump opened debate with fireworks Tuesday as Democrats accused Senate Majority Leader Mitch McConnell of fomenting a "cover-up" with his proposed rules for the process.

The Republican McConnell proposed a set of ground rules that would place strict restrictions on witnesses and evidence for the first stage and move the trial quickly ahead, saying he would summarily block any Democratic attempts to change his rules.

"The basic structure we're proposing is just as eminently fair and even-handed," McConnell.

"There is no reason why the vote on this resolution ought to be remotely partisan," he said.