Trump's campaign is set to sue the Philadelphia administration on charges that it prohibits presidential supporters from overseeing the voting process.

"In the event that your municipal government prohibits us from monitoring the voting process, we will resort to court intervention," said the lawyer representing the US President Donald Trump's re-election campaign today.

The threat of a lawsuit came from the Trump campaign after Mike Roman, a Trump supporter who claimed to be a "election observer", was invited out of Philadelphia City Hall.

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Thea McDonald, deputy press secretary for Trump's campaign, accused Philadelphia electoral officials of "deliberately evading accountability and hiding their voting system".

"Trump's campaign, and of course Biden's campaign too, has the right to send observers to observe the voting process to ensure polling stations are running properly and all rules are in place.

Philadelphia city election officials have not yet commented on the information.

During his first presidential debate on the evening of September 29, President Trump said that the Philadelphia voting process was a "big problem".

Voters in some US states can vote early by mail or to a polling place in September, even though the election date is set on November 3.