Labour opponents in the Glasgow East by-election have poured scorn on the party's "lost weekend" after it failed to select a candidate.
Holyrood MSP Margaret Curran could now be endorsed to contest the Westminster seat on 24 July, after frontrunner George Ryan pulled out of the race.
SNP leader Alex Salmond said Labour was in "complete meltdown".
Former Scottish Labour spin doctor Lorraine Davidson said defeat would spell the end for the prime minister.
The Glasgow East by-election was sparked by the resignation on health grounds of David Marshall, who had a majority of more than 13,500.
Labour had hoped to select local councillor George Ryan as its candidate on Friday night but he failed to turn up for the meeting.
He pulled out of the contest to prevent "pressures" on his family life.
Former Scottish cabinet minister Margaret Curran, currently a Glasgow MSP, then announced she was adding her name to the shortlist.
A final decision on the candidate is expected to be taken on Monday evening.
However, the confusion will do little to raise the party's morale after heavy by-election losses in Crewe and Henley, disastrous poll ratings, and the resignation of its leader in Scotland, Wendy Alexander.
Defeat in the previously rock-solid Labour heartland of Glasgow East could put Mr Brown's leadership under unbearable strain.
Former Scottish Labour spin doctor Lorraine Davidson says Gordon Brown's future is at stake in Glasgow
Ms Davidson, who now works as a political columnist and journalist, said: "I think it's as simple as this: if Labour loses the Glasgow East by-election I think Gordon Brown is finished.
"Because it basically means that anyone in the PLP (Parliamentary Labour Party) with a majority of less than 13,500 is going to say, 'if Gordon Brown's the leader, we're away as well'.
"You're really talking the bulk of the PLP. It's complete and utter meltdown.
"So if Glasgow East is lost I think you will see a challenge to Gordon Brown. I just do not see how he can survive it."
However, Ms Davidson added that she did not feel there was the same desire to give the government a "good kicking" as she had detected at recent by-election defeats.
Political opponents immediately seized on the situation as evidence that Labour was in "meltdown".
Mr Salmond, Scotland's first minister, said his party was off to a "flying start" after Labour's "lost weekend".
He said: "They don't have a leader in Scotland, they don't have a candidate in Glasgow East, and they have a prime minister who refuses to come to the constituency."
Liberal Democrat campaign chairman, Robert Brown said: "Nobody knows what they stand for any more and, therefore, it is no surprise that they are finding it difficult to find a candidate."
Scottish Tory leader Annabel Goldie claimed Ms Curran had been "pressganged" into standing.
But Ms Curran, Labour's health spokeswoman at Holyrood, said: "I have decided to do this because I'm deeply committed to the communities of the east end of Glasgow.
"I am hoping to be the Labour nominee in order that we have a spirited campaign in the by-election."
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