The party urged ITV chief executive Dame Carolyn McCall not to help the leaders of the two largest parties "stitch up the debates" ahead of the December 12 vote.
Lib Dem president Baroness Brinton wrote to the broadcaster's chief on Saturday arguing for Ms Swinson's inclusion so an unequivocal Remain-backing voice is heard in the November 19 debate.
ITV said it "intends to offer viewers comprehensive and fairly balanced general election coverage," pointing to a live debate to which leaders of seven parties have been invited.
Baroness Brinton wrote: "The voters of this country deserve to hear from a Remainer on the debate stage, not just from the two men who want to deliver Brexit.
"A debate between just them offers no real alternative and stifles the conversation.
"Boris Johnson and Jeremy Corbyn are running scared of debating Jo Swinson and broadcasters should not help them stitch up the debates."
She stressed rules from the regulator Ofcom that due weight must be given to coverage of political parties during an election period, and also pressed the importance of representation of women during the debate.
She wrote: "We have seen an exodus of women from politics in the run-up to this election. I want to do everything in my power to end this. I'm urging you to do the same."
She argued, in part, that the Lib Dems deserve a place in the debate because they won more votes than the two main parties in the European Parliament election in May.
The broadcaster said in a statement on Saturday: "ITV intends to offer viewers comprehensive and fairly balanced general election coverage.
"This involves a wide range of programming, including a live debate in which seven party leaders are invited to take part, as well as a live debate between the Labour and Conservative leaders."