But the First Minister did suggest the Prime Minister would be in a "difficult position" if it was judged he did not "try his hardest" in the campaign.
The comments came after it emerged the pro-independence side had received £2.5 million in the past year from EuroMillions lottery winners Chris and Colin Weir.
Both campaigns also claimed a new poll showed the momentum was with them.
In a radio interview, Mr Salmond refused to say that Mr Cameron would have to resign if he became the prime minister who presided over the breakup of the Union.
But he reiterated his call for the Tory leader to agree to a head-to-head TV debate on independence.
And he added: "I think David Cameron underrates the difficulty he is in. If he were to lose the referendum ... I think everybody would now accept that ... he would be in a particularly difficult position if he doesn't enter the debate as I have challenged him to do."
Mr Salmond also said he would not stand down if his side lost the vote.
"It's always been the Scottish people who have decided whether I am in post or not," he said.
Earlier this week Mr Cameron insisted he would not resign if there was a Yes vote, adding that his name did not appear on the referendum ballot paper.
The pro-Union Better Together yesterday seized on the Weir donation, saying it showed the Yes campaign relied on a single SNP source for nearly 80% of its funding.
Meanwhile, a new poll found 54% of those asked plan to vote No, compared to 34% who said they would vote Yes and 12% who were still undecided.
In another development, the Scotch Whisky Association has warned a Yes vote could hurt the industry.
And it emerged the Orange Order is reportedly planning a "massive parade" in support of the Union on the eve of the referendum vote.