The High Court is considering separate legal challenges from the Lib Dems and SNP over their exclusion from ITV’s general election debate.
Its head-to-head between Conservative leader Boris Johnson and Labour leader Jeremy Corbyn takes place on Tuesday.
But the SNP and Lib Dems say it is unfair not to invite them to take part.
The Lib Dems have also sent a legal letter to the BBC over its decision not to include leader Jo Swinson in a debate on 6 December.
The SNP said it expected the High Court to decide on Monday whether the two legal challenges should be heard together, and a ruling is expected later in the day.
The Liberal Democrats argue that the Conservatives and Labour are both pro-Brexit parties and it is wrong to exclude “a voice of Remain” – and the only female candidate for prime minister – from the head-to-head debates.
Arriving at the High Court in London, Lib Dem chairman Sal Brinton said ITV’s decision not to include Ms Swinson in the TV debate was “weak and shameful”.
She said: “No TV executive should have the power to decide which voices are represented to the British people, and no individual should have it dictated to them, who their vote should be between.”
“Jo Swinson’s Liberal Democrats are the voice of Remain. The appetite in this country to hear that argument, amassed with the existing support in the UK for remaining in the European Union, is undeniable.”
Also arriving at court, SNP Westminster leader Ian Blackford said he hoped that “sense prevails”.
“This is really important because lots of people make their minds up on how they’re going to vote in the election campaign based on these debates,” he said.
He said the SNP should be included as it was the third-largest party in Westminster and by membership in the UK, so its “very distinctive voice” should be heard.