I've linked previously to the instructive work from Populus comparing the poll standing of Brown's government and that of Major's in 1992-1997. Whilst he prefers to focus on a slightly different element of the Major premiership - i.e. the bit before an election victory - it seems that the comparison between Brown and Major is one Brown himself takes to heart, as apparently "Shaun [Woodward] has become an increasingly important figure in Team Brown" because Woodward was Major's communications director in the run-up to the 1992 election, which we won after a number of by-election defeats, and "Gordon finds that analogy comforting". Still, whilst Brown does have backbenchers openly calling for him to go, he has a healthy majority and he has no hard core of habitual, principled opponents a la Major's "b**tards".
As The Times has it, perhaps the apposite comparison is with Gordon and Thatcher. After all, it was "the loss of the safe Tory seats of Mid-Staffs and Eastbourne that confirmed that she had become an electoral liability..." [a point I for one don't in fact concede, but...] whilst "For Gordon Brown it has taken the London Mayoral Battle, Crewe & Nantwich and now Glasgow East to prove tha the prime minister is an election loser." Then again, "in just one year as prime minister, Mr Brown has reached the point that it took Mrs Thatcher 11 years to get to." An instructive point.
I don't know if Brown is really aptly compared to either Thatcher or Major. A reasonable point to make on both fronts is that both of our most recent PMs struggled with economic downturns; 80% of us now believe that the country is heading for a recession, a high enough result for it to become a self-fulfilling prophecy as people trim their behaviour accordingly.
In any case, I think that the best comparison is probably with the person with whom Brown has always most loathed comparison - Tony Blair. I often wonder what would be different, had Blair stayed as he promised to. For example, in the best of the Labour-insider-sticks-knife-into-PM columns in the aftermath of the by-election, an incisive and insightful article which I urge you to read, Lance Price wrote that he believed that "under a different leader Labour would not have lost Glasgow East" - perhaps one A C L Blair would have fitted the bill...