The battle over voting rights in the US is a drama that's playing out in the Congress and state legislatures across the country.

In Philadelphia on Tuesday, Joe Biden gave a fiery speech, warning that American democracy is facing its greatest threat since the Civil War.

"There's an unfolding assault taking place in America today, an attempt to suppress and subvert the right to vote and fair and free elections," the president said.

Biden condemned recent efforts in 17 Republican-controlled states to pass laws curtailing when and where their citizens can vote - laws conservatives have framed as a response to allegations of voting fraud repeatedly made by Donald Trump in the months after his presidential defeat.

Many Republicans now believe the election was stolen from Trump - an assertion Biden called "the big lie" and one that has not been substantiated by concrete evidence.