There is a huge amount at stake for both David Cameron and Nick Clegg, but it is perhaps the Liberal Democrat leader who has most to lose.
Having become the fall-guy for the coalition’s most unpopular policies, and then switched to backing AV after previously describing it as a “miserable little compromise”, his authority will suffer a severe blow if the Yes campaign is defeated.
This is an inauspicious political backdrop but in launching the push for change in Leeds yesterday Mr Clegg made a salient point. Under the first-past-the-post system, too many MPs have seen a Commons seat as a job for life. They have fallen into complacency or, as the expenses scandal showed, corruption. AV would force politicians to serve their whole communities, rather than simply party loyalists and swing voters.
What it could also bring, however, is confusion and backroom political horse-trading in the aftermath of each General Election. Centuries of relatively stable government could be replaced by years of coalition with no party winning a mandate to change Britain. That fear will make Mr Cameron’s job slightly easier, because his No campaign can tap into a natural preference for the status quo.
While the Prime Minister will be damaged if his campaign is defeated, he is a big enough political figure to shrug it off with a Blairite claim that “the people have made their choice” and simply move on. For Mr Clegg, that will be a harder move to perform.