Ed Miliband and Nicola Sturgeon are "at the altar, and preparing to sign the register" on a post-election deal, according to Scottish Conservative leader Ruth Davidson.
Sturgeon made the Labour leader an offer to join forces to help him become Prime Minister during the TV debates.
But David Cameron has warned there would be more borrowing and higher taxes if Miliband goes ahead with the deal.
David Cameron has warned there would be borrowing and taxes if Ed Miliband does a deal with the SNP to become Prime Minister.
The Tory leader said Miliband had failed to rule out a vote-by-vote deal with Nicola Sturgeon after she offered an olive branch during last night's "challengers' debate".
Cameron wrote on Twitter:
The Ukip leader said he did not "lose my rag" when he attacked the audience during last night's televised debate for their "left-wing bias."
We watched undecided voters as they watched the debate hoping it might help them make up their minds.
Ed Miliband appeared to turn down Nicola Sturgeon's latest offer as the leaders of five "challenger" parties took part in a heated debate.