A headline in today’s Age has “Labor-Greens feud verging on war”. That was certainly the message from Greens leader Bob Brown in Friday’s Australian, in an op-ed piece titled “Greens won’t preference the ALP”.

Brown might not be responsible for the heading, but the substance was trenchant, attacking Kevin Rudd for his support of the government’s line on the Haneef case, the Northern Territory intervention and the coal industry, and accusing him of engaging “in a bidding war to subsidise the logging industry”.

In the circumstances, says Brown, “the choice between Labor and the Coalition is becoming more like choosing between fawn and beige… Rudd is making it harder to recommend Greens voters preference Labor.”

Most commentary on the Labor-Greens quarrel has taken the position that this is an empty threat, for two reasons. Firstly because the Greens’ power to influence their voters is limited; running “open tickets” typically leads to only a slight reduction in the preference flow to Labor.

And secondly because Brown and his colleagues have as much interest as anyone in seeing the end of the Howard government, and are unlikely to do anything to jeopardise that outcome.

But what’s interesting is that both those reasons are specific to the House of Representatives. Neither applies to Greens preferences in the Senate.

Senate preferences mostly go where they’re directed, because of the group ticket voting system. Even among Greens voters, who are more independent than most, the vast majority vote “above the line” – more than 80% in every mainland state, and more than 90% in New South Wales and Victoria. And Senate preferences have no effect on who wins government, only on what sort of upper house the government will face.

Of course Brown will want to avoid the Howard government being returned with a Senate majority. But realistically, that’s not what’s at issue in the Senate election: if the government has enough votes to retain government, it will almost certainly have enough to retain its majority in the Senate.

The choice is between a Rudd government with a hostile Senate and one where minor parties (among them the Greens) have the balance of power.

No doubt the Greens would prefer the latter, but the former would have its advantages as well – it might lead to an early double dissolution, in which the Greens could expect to increase their representation.

If Brown is really looking for a way to put Labor on notice, then a more discriminating approach to Senate preferences would be an obvious place to start.