However, Alex Salmond, the leader of the pro-independence Scottish National Party (SNP), enjoys a reputation as a canny campaigner who has unexpectedly won elections in the past.
"It's the last real public chance to reach a large audience," Patrick Brione, director of research at pollster Survation, said of Monday's TV debate.
"Salmond is very much the underdog at the moment, so he really needs to pull off an impressive performance."
Read MoreScottishindependence is bad for business
The debate is expected to centre on three issues: if and how an independent Scotland could keep the pound, how many barrels of oil are left in the North Sea, and whether Scotland's publicly funded health service would be better off in a breakaway state.
Salmond unexpectedly failed to dominate the first such TV debate, on Aug. 5, in which Alistair Darling, the leader of the anti-independence "Better Together" campaign, put him on the spot over the issue of currency arrangements in an independent Scotland.
The question is one that opponents of independence have long pushed for an answer to. Yet Salmond seemed blindsided by Darling's persistence and failed to spell out what his "plan B" would be if the British government refused to formally share the pound, his preferred option.
Read MoreGambler bets$670,000 against Scots independence
All three major UK-wide parties have ruled out such a union; Salmond predicts their position will change if there's a "yes" vote.
"Going into the previous debate he was the favourite, and as a result of that people thought that he lost the debate," Survation's Brione said of Salmond.
"Darling benefited very much from people not expecting him to do well going into the first debate. His performance, however good as it was, was made to look better."