Labour holds 345 seats in Parliament and the party could still cling to power even if it is handed a humiliating third-place finish in the popular vote Thursday. Brown told The Observer newspaper that his party was the one facing the longest odds.
"We are the underdog," he said. "But we're fighting with every inch of our being."
The Observer threw its support behind the insurgent Liberal Democrats, who shot to the top of the polls after Britain's first U.S.-style televised debates. In its endorsement, the paper hailed leader Nick Clegg as "the candidate of change" and backed his call for a revamp of the British electoral system, whose arithmetic tends to favor Labour and the opposition Conservative Party.
"Opinion polls throughout the campaign suggests this country want the Lib Dems to take a place of equal standing alongside the other main parties," the paper said in an editorial. "A grossly unfair voting system has historically deprived them of that right."
The Observer's is the latest newspaper endorsement to go to Brown's rivals in what has been an especially difficult campaign for the prime minister. The weekly's sister paper, The Guardian, has also cast its lot with the Lib Dems. The Times, along with other Rupert Murdoch-owned publications such as The Sun and The News of the World, are backing the Tories.
Others, such as The People and The Independent on Sunday, which have urged voters to support whichever party is most likely to keep the Tories out of power, have nonetheless stopped short of outright support for Brown.
Other segments of the British press continued to pile the pressure on the prime minister. The populist Mail on Sunday publishing a four-page interview with the retiree who Gordon Brown was overheard calling a "bigoted woman" after she complained to him about immigration at a campaign stop in northern England.
The comments, captured on an open microphone as Brown left in his car, threatened to derail his campaign and forced him to race to Gillian Duffy's home, where he spent nearly 45 minutes apologizing for the gaffe.
Duffy told the Mail she wasn't impressed.
"Sorry is a very easy word, isn't it," she was quoted as saying, adding that she would not be casting her vote for Brown — or anyone else — in the election.
Still, Brown was making good on his promise to step up his campaign. The prime minister was hitting no fewer than 10 campaign stops across London Sunday, joining congregants in a prayer at a local church, visiting a community center and campaigning with his transport minister Sadiq Khan, whose supporters sported Barack Obama-inspired "Yes We Khan" T-shirts.
His opponents weren't taking Sunday off either: Tory leader David Cameron made an appearance at a question-and answer-session in the coastal town of Newquay, while Clegg addressed supporters across northern England.