Labor had a horror month of leadership farce and superannuation policy self-harm and it's a month closer to the federal election that their team is set to win, but it's the Coalition voters who turned negative last month and dragged the Westpac/Melbourne Institute consumer sentiment index down by 5.1 per cent.

As the accompanying graph shows, the gap in consumer confidence perceptions between Labor and Coalition voters is again widening after a period of rapprochement from the record July chasm – back when the sky was supposed to be falling with the start of the carbon tax and the $100 leg of lamb.

Coalition voters' consumer sentiment index dropped 5.8 points to stand at 95.8 this month – below the 100 break-even mark when the numbers of pessimists and optimists are equally matched. Labor voters' consumer sentiment firmed 0.9 to a chirpy 127.9.

For the past couple of years, Coalition voters have been the main drivers of consumer sentiment gyrations while Labor voters' confident economic outlook has remained relatively consistent. Given the size of the voting intentions demographic subset, this variation between the two classes is the most influential and most intriguing, yet the Westpac commentary with the release of main index number doesn't mention it.

Westpac chief economist Bill Evans offers various possible reasons for the index turning south this month – Cyprus, the stock market correction, talk of interest rates perhaps rising – but it is curious that only Coalition voters are moved.

It's not hard to propose excuses for some of the subset movements. By far the biggest demographic swing this month was the outlook of tenants plunging 18.7 points to 93.6: rising rents and news of higher housing prices will do that. The outlook of mortgagers and outright owners only dipped 2.5 and 1.3 points respectively and remained in positive territory.

The extent of whip-sawing in some of the demographic subsets suggests the danger of relatively small sample sizes, but the voter intention numbers should be fat enough to have weight.

While Coalition voters apparently were concentrating on the possible negatives, recent economic news has been reasonably good. Trade balance, retail sales, jobs growth and housing approvals have all been positive (although the too-good-to-be-true labor force numbers will be tested later today). The Reserve Bank left interest rates unchanged again, but maintains an easing bias in its rhetoric with no suggestion that the current historically low rates might turn up any time soon.

Maybe Joel Fitzgibbon did it with his suggestion that, if you climb the ladder of opportunity to household income of $250,000 a year, you'll still be struggling.

Michael Pascoe is a BusinessDay contributing editor.