As the Michigan primary approaches, the race between Mitt Romney and Rick Santorum remains tight. So tight, in fact, that Santorum is reaching out to a very unusual demographic indeed for the Republican primaries: Democratic voters.

It is a bizarre tactic; so much so that it was initially speculated that the call was not really paid for by the Santorum campaign, but was an attempt by his opponents to damage him. However, a spokesman has confirmed that the calls were indeed paid for by Santorum, saying that "if we can get the Reagan Democrats in the primary, we can get them in the general."

What's most interesting is that it appears to criticise Romney from the left -- "we're not going to let Romney get away with it", says the robotic voice, criticising Romney for his links to "billionaire bankers".

It's a particularly cynical tactic given that Santorum is, for the most part, much further to the right than Romney, the former Massachusetts governor, and has staked his campaign on the ideological right wing of the GOP.

This is illustrated by the latest ABC News/Washington Post poll, which shows that just 38 per cent of voters who identify themselves as "very" conservative hold a favourable opinion of Romney -- down 14 points from last week -- while 60 per cent of this group have a positive view of Santorum.

The hard core on the right has proved intransigent in its opposition to Romney, who is a moderate conservative. However, as Alexander Burns notes at Politico, while this latest tactic may help tip the vote Santorum's way, it might be ultimately counter-productive:

[I]t's helpful for Romney that the Santorum campaign has acknowledged actively encouraging Democrats to cross party lines and vote against Romney in the primary. That way, if Santorum wins the state by a narrow margin, Romney will be able to argue that it's because of Democratic sabotage rather than tenacious conservative resistance.

Whether the tactic even helps Santorum to win the vote in the first place remains to be seen tonight.