One of the quiet crises of David Cameron's leadership is the continuing decline in Conservative Party membership. A study by the House of Commons Library recently found it had fallen to a modern low of 177,000. Now, a new ConservativeHome survey (previewed in today's Independent) suggests even this figure is generous, with membership estimated at between 130,000 and 170,000, a decline of around 50 per cent since Cameron became leader in 2005.

The Tories are far from the only party afflicted by falling membership. In 1983, nearly four per cent of the electorate belonged to one of the three main parties. Now, just one per cent do, one of the lowest rates of party membership in Europe. Although Labour membership has risen by 31,000 to 187,000 since Ed Miliband became leader, this remains far below the peak of 405,000 seen under Tony Blair in 1997. The Liberal Democrats, meanwhile, have seen membership fall by 5,000 since the general election to 60,000, down from a peak of 101,000 in 1994. But it is the Tories, who once boasted a membership in excess of three million (see graph), who have suffered the most rapid decline. Should the trend continue, membership will soon fall below the psychologically significant 100,000 mark.

The Daily Mail's Andrew Pierce has previously attributed the decline to Cameron's prominent support for gay marriage, reporting that thousands "ripped up their membership cards and refused to renew their subscriptions." He added:

The alarm bells sounded in the Tory HQ, which in January launched a national appeal to try to persuade waverers to return to the fold. The appeal was a dismal failure.

The constraints of the coalition mean that Cameron can do little to woo traditionalists back to the fold. ConservativeHome editor Tim Montgomerie said: "Cameron's compromises on traditional Tory beliefs and the failure of those compromises to deliver a parliamentary majority mean he's upsetting both kinds of grassroots member."

Cameron's failure to retain existing members or to recruit new ones is yet another reason why the odds are against a Tory majority in 2015.