The Maritime Union's Chris Brown is set to win preselection as Labor's candidate in the federal seat of Fremantle after he was overwhelmingly endorsed in a ballot of the left faction.

Mr Brown won the vote decisively 76-5, including the backing of the United Voice union, which had initially supported his rival, Josh Wilson.

Mr Wilson is the deputy Fremantle mayor and staffer to outgoing MP Melissa Parke.

Ms Parke and former WA premier Carmen Lawrence had previously warned Fremantle could be lost to the Greens in a voter backlash if a Maritime Union of Australia (MUA) candidate was installed.

But it is understood United Voice decided not to take on the MUA and other blue collar unions because it prioritised unity and a strongly backed candidate, rather than risk the left tearing itself apart.

The left faction will now almost uniformly vote for Mr Brown when the ALP state executive meets on Monday.

He is also believed to have the support of the right faction unions, the Shop Distributive and Allied Employees' Association, the Australian Workers Union and the Transport Workers Union.

The state executive ballot counts for 75 per cent of the overall vote, while Sunday's local elector's ballot, where Mr Wilson is expected to perform strongly, counts for only 25 per cent.

In other states, it is a 50-50 split.

That means Mr Brown is virtually certain to win ALP preselection for Fremantle, barring an unlikely intervention by the federal branch of the party.

In January, MUA state secretary Christy Cain said Fremantle should be an MUA seat.

Fremantle Mayor Brad Pettit had earlier indicated he could run as a Green or independent if an MUA candidate was chosen, but has now dropped the idea after the Greens announced lawyer Kate Davis as their candidate.

Mr Brown declined to comment.

Mr Wilson urged people to make their voices heard at the local electors ballot.

"It's critically important that local Labor members take this rare opportunity to participate in the selection of their Labor candidate, and I am determined to see this process through so that can occur," he said.

"You have to fight for what you believe in and I know rank and file Labor people want to have their say."