The SNP is aiming to raise a 50,000 fighting fund over 48 hours to pay for the action at the Court of Session in Edinburgh.
The party is angry that leader Alex Salmond has been excluded from Thursday's BBC debate between Gordon Brown, David Cameron and Nick Clegg.
They intend to lodge papers with the court tomorrow, seeking a review of the broadcaster's position to "ensure that Scottish interests are properly represented".
The proposed action would seek to ensure that the debate is broadcast in Scotland with the nation's political make-up "fairly" reflected – either by having SNP representation in the debate or through an agreement to have a further leaders' debate.
Mr Salmond said: "The decision by the BBC, who are meant to be Scotland's national broadcaster, paid for by our licence fees, not to have the country's political make-up properly represented next Thursday is a democratic disgrace."
Labour branded the SNP's legal bid "desperate", saying it was revealed on the same day that Mr Salmond appeared in the second of three televised debates discussing Scottish issues.