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.
Ed Miliband has appeared to suggest that the United States' decision not to pursue plans for airstrikes against Syria in 2013 was a consequence of his opposition to the move.
Speaking during this evening's BBC debate, he said: "In 2013, David Cameron came to me [with] Nick Clegg and they said that they wanted to bomb Syria.
"They hadn't thought through the consequences, it wasn't a last resort, so I said 'no'.
"And as a result, the British parliament said 'no', and then President Obama decided not to go ahead with that action."
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.