• Joe Biden's campaign has attacked Republicans for blocking legislation intended to block foreign interference in US elections.
• US intelligence agencies found that Russia helped Donald Trump win the 2016 election.
• Republicans were accused of removing language from the bill that would make offers of help illegal.

Presumptive Democratic presidential nominee Joe Biden's campaign on Tuesday criticised US Senate Republicans for blocking legislation to require campaigns to report offers of foreign election help to the FBI, saying it could open the door to interference in the 2020 contest.

Senator Mark Warner, the ranking Democrat on the Senate Intelligence Committee, said Republicans had stripped language he added to an intelligence bill intended to prevent actions like those by Russia that US intelligence services concluded were an attempt to undercut Republican President Donald Trump's rival in the 2016 campaign.

"This action today can only raise questions about partisan motivation in leaving open the channels for hostile foreign government intervention in our election process," Biden campaign spokesperson Andrew Bates said in a statement.

US law already makes it illegal for campaigns to accept anything of value from foreign sources. Warner's measure would have compelled them to disclose any offers of aid from a foreign national.

Warner said Republican leaders removed the provision ahead of a Senate debate on the intelligence bill, which is being incorporated into the annual National Defence Authorisation Act that sets defence policy.

Bounty allegations

A spokesperson for Senator Marco Rubio, the intelligence committee's interim chair, did not immediately respond to a request for comment.

Multiple US intelligence agencies found that Russia acted to help Trump, a charge that Russia denies and Trump has repeatedly labelled a "hoax".

Bates also noted recent reports that the Trump administration took no action against Russia despite intelligence suggesting it paid bounties to Taliban insurgents in Afghanistan to kill US troops.

Trump has said he was not briefed on the bounty allegations.

Warner first introduced the legislation last year after Special Counsel Robert Mueller's investigation into Russian interference.

The measure was then blocked by Republican Senator Marsha Blackburn, who called it a "stunt".