This morning’s Australian brings news of another Galaxy poll — commissioned by the same outfit, GetUp!, that paid for last week’s poll on gay marriage. This time it’s about Senate voting intention: GetUp! is backing a campaign to try to deprive the Coalition of its Senate majority at this year’s election.

The poll shows a drop of 11% in the Coalition vote, with most of it going to the minor parties (Labor is up only 3%). That could well be right, and although it is rather different from a Morgan poll released last month (which showed Labor’s vote up 10.5%), either way would probably result in the Coalition losing Senate control.

But in addition to doubts one might have about commissioned polls in general, and Galaxy in particular, there’s a further reason to be cautious about these results. Senate-specific opinion polls have a generally poor record; for one thing, they tend to exaggerate the support of minor parties. Prior to the last federal election William Bowe, the Poll Bludger, tried to explain why:

Respondents would have been asked about Senate voting intention after they had already given an answer for the House, and many would have felt compelled to nominate a different party because they did not wish to appear unsophisticated. In many cases the Democrats would have been the first name that entered their heads.

We don’t know if Galaxy asked a question about the House of Reps first, so that particular contamination might not have been present. But the basic problem is still there: when people are asked a poll question about the Senate, they think about the Senate separately.

When they actually go into the polling booth, however, most of them don’t do that; they just follow the how-to-vote card they’ve been given. That’s why, paradoxically, polls for House of Representatives voting intention are actually a better guide to Senate results than polls for Senate voting intention.

If the GetUp! campaign succeeds in getting voters to think about Senate control as an election issue, then it’s possible that will change. But my guess is that such constitutional concerns will remain confined to a small minority.