In response to the latest Ipsos MORI/Reuters poll, which put Labour and the Tories neck and neck on 37 per cent, he writes:
After losing in October 1951, Labour had pulled ahead by January 1952, but it didn't stop the Conservatives enjoying 13 years in government. In 1970, after a June election, Labour were level by October; that didn't stop Ted Heath polling more votes four years later, even if he didn't secure enough seats to cling on.
He adds that Labour "led Mrs T's cutting government within a month of the  election", but was still out of power for the next 18 years. "Don't get too excited, we've been here before," is Cowley's message.
But there are good reasons to believe that 2010 will prove atypical. For a start, the cuts planned by the coalition, the largest since the Geddes Axe of the 1920s, are greater than anything we saw under the Iron Lady.
Many on the left are unaware that, despite her neoliberal ideology, spending actually rose during Margaret Thatcher's premiership. While education and health were neglected, spending on defence, law and order and welfare payments (thanks to mass unemployment) continued to grow.
Overall, public spending under Thatcher -- from 1978-79 to 1989-90 -- rose by 1.1 per cent a year on average.
There has, therefore, never been in living memory an austerity drive of the sort planned by the coalition. Figures on the left and the right are agreed that those 25 per cent cuts will make David Cameron's government fantastically unpopular.
The coalition hasn't even announced, let alone implemented, the key cuts, but already this week it has attracted the ire of the trade unions, the police and the defence establishment. No wonder one Liberal Democrat cabinet minister has predicted that support for his party will fall to 5 per cent and the Tories to 25 per cent.
Cowley's analysis is an antidote to Labour complacency, but fails to factor in the historically extraordinary cuts we're about to see.