The legalisation of marijuana is shaping as a key issue in the re-run Senate election in Western Australia because voting preference experts have identified Help End Marijuana Prohibition (HEMP), a micro party with 6000 members, as having the best chance of success among the minor party alliance.

It is also causing headaches for the WA Greens Party as HEMP has abandoned preference deals with it and has preferenced Labor - a reversal of its position at the September election.

The party says the Greens backed out of a promise to host a drug summit in Canberra in return for HEMP's preferences.

And it has accused the WA Greens senator Scott Ludlam of being a social conservative on the issue of legalising marijuana and other drugs.