Left-wing minor parties are threatening to withhold preferences from the Greens at the re-run WA Senate election in retaliation for what they say were dirty tactics by the Greens at last year's Federal poll.

As Parliament ended the week with the major parties setting the scene ahead of the first full week of campaigning in WA, Greens senator Scott Ludlam is in peril, with Labor preferences also in doubt.

Labor is set to capitalise on the anger of several progressive parties which preferenced the Greens at last year's election.

Sex Party president Fiona Patten said many members were still furious the Greens ran a national social media campaign in the lead-up to the September 7 poll, claiming that a vote for the Sex Party would help elect Pauline Hanson to a NSW Senate seat.

Ms Patten said the "smear" would be kept in mind when the party settled on its preferences.

"As one of the few progressive parties, we would like to have seen them support us rather than do a nasty social media campaign," she said.

Help End Marijuana Prohibition Party national campaign director James Moylan said the party was "disaffected" with the Greens, especially over their failure to keep an election promise to hold a drug summit.

Preferences will be crucial. With Labor polling just under two Senate quotas last year, it is pursuing preferences to ensure its second candidate Louise Pratt holds her seat. But if Labor gets just over two quotas, its surplus votes become crucial, especially for Senator Ludlam because the Greens are unlikely to get a quota in their own right.

The Greens would also welcome minor party preferences.

At last year's election, Labor preferenced the Greens second, but the Greens preferenced nine micro-parties ahead of Labor.

_The West Australian _ understands there is a growing mood within Labor not to give the Greens the second preference and direct them via other left-leaning parties before the Greens. Boosting this sentiment is the view the Greens have nothing to offer Labor this time because no Lower House seats are at stake.

Greens leader Christine Milne warned control of the Senate was at stake if Labor did not preference the Greens.

"This is all about their internal factions trying to shut the Greens out of Parliament," she said.

Senator Milne urged voters to look carefully at where their preferences were going because the electoral system had been gamed through stitched-up deals.