Jim Murphy's performance in the Scottish leaders' debate last night was confident, polished and persuasive. It was also hugely damaging to Labour's chances of forming the next government.
Why is this? Well, as things stand, Labour are highly unlikely to win an overall majority at the election. According to most forecasts it is also likely that the Tories will be the largest party in a hung parliament.
If this turns out to be the case then you might expect Ed Miliband to have no chance of becoming prime minister. You would be wrong. In a parliamentary system, it is not the party which has the largest number of seats, but the party which is most able to pass a majority in that parliament, which gets to govern.
If current polls are correct, that party is Labour. The Conservatives know this, which is why they have spent the past few months trying to delegitimise the idea of any kind of post-election arrangement between Labour and the SNP.
Last night, Jim Murphy played right into their hands.
Asked repeatedly by Nicola Sturgeon whether Labour would accept SNP support, Murphy replied: "We don't need your help".
When Sturgeon pointed out that a minority Labour party would almost certainly need SNP votes in order to keep David Cameron out of Downing Street, Murphy replied that in those circumstances it would be the Tories' right to form the government.
"When was the last time the biggest party didn't form the government?" he asked, before adding that in 2010 "the fact that David Cameron was leading the biggest party meant that he got to form the government." He went on to describe the prospect of a Conservative government in those circumstances as an "unstoppable force".
It was like watching an athlete, just seconds away from crossing the finishing line, stopping on his heels to tie his shoe laces up. It was a moment of unbelievable self-harm and it could end up costing Ed Miliband his only chance of becoming prime minister.
Before last night Labour held a massive inbuilt advantage over the Conservative party. Last night Jim Murphy handed that advantage right back to them.
If Labour fails to win most seats on May 7th, we can expect Jim Murphy's words to be played back repeatedly in the crucial hours and days that follow the result. The Tories and their supporters in the press will use them to discredit the idea of any kind of minority parliamentary coalition. Ed Miliband may still be able to form a government, but it will immediately be perceived as illegitimate. Jim Murphy will share a large part of the blame for that.
Of course the Tories' entire argument is false. The UK is not a presidential system. If the majority of MPs in parliament are opposed to a Conservative government and have stood on a ticket of preventing such a government, then it is entirely legitimate for them to vote to do so. And if Scots vote overwhelmingly for the SNP, then it is also entirely legitimate that they should have a decisive role in who becomes the next prime minister.
But politics is as much about perception as raw electoral mathematics and right now the Tories are winning the perception battle hands-down.
Jim Murphy's debate performance may well have helped his party's chances on May 7th, but he has massively damaged their chances in the days that follow.