Scott Ludlam's ''welcome to Western Australia'' speech against Prime Minister Tony Abbott lit up social media and made him an instant hero of Gen Y.

But the Greens senator's campaign for re-election in WA has been hit by claims he is actually a social conservative on one important issue to many young people: the legalisation of marijuana and other drugs.

Facing a legalisation debate: Senator Scott Ludlam. Credit:Penny Bradfield

The legalisation debate is shaping as critical in WA because preference experts have identified the Help End Marijuana Prohibition, a micro party with 6000 members, as the best chance of success among the minor party alliance.

HEMP's lead WA candidate, Jim Moylan, blasted the Greens as ''a joke on cannabis law reform''. ''The Greens take votes away from us because people just assume they are for cannabis law reform,'' he said.