At a special conference this Saturday, Labour will vote on plans for expanding the Senedd to make it more representative and changing how it is elected.
Cardiff Bay will see whether or not the number of Members (MSs) of the Senedd should rise from 60 to 96.
Voters would choose one party list, while larger constituencies would elect six MSs under the new electoral system.
Three Labour-supporting unions warned that the plans could make it more difficult for the party's survival.
Since 1999, Labour has been the government of Wales and has never been in opposition.
On Saturday, delegates from Welsh Labour groups and associated bodies - including trade unionism - will vote for the proposed changes.
GMB, Community, and Usdaw are opposed, but the largest unions in Wales, Unison and Unite, are in favor.
The conference voting rules for Labour include a significant part on the membership.
Opposition to the reforms is coming from Labour groups in Swansea East and Llanelli, as well as Rhondda. Chris Bryant, the MP, stated that it would make politicians "less connected with voters".
Mike Hedges, a Labour Swansea East Senedd Member, said that he expected the conference "overwhelmingly to support the change", but also expressed doubts about the "very big constituencies".
Plaid Cymru and the Welsh government reached a deal in May with Plaid Cymru to bring about the changes in time for next year's Senedd elections in 2026.
Parties would be subject to a gender-quota in the new system, which aims to encourage better representation of women.
The scheme would see the Senedd pair constituencies that were proposed for Westminster during the Boundary Commission for Wales' ongoing review, which is not expected until 2023.
According to the commission, it will confirm 32 constituencies' boundaries. This would mean that the Senedd would have 16 members at the next election and six Senedd members.
Currently, there are 40 Senedd constituencies that have been elected by first past the post. Five regions were elected using a list system.
Mid-June saw a letter addressed to constituency Labour parties expressing opposition to the GMB, the steelworkers union Community and Usdaw which focuses on retail.
They expressed concern about "any changes [to] the Senedd electoral process that would make it harder to secure a Welsh Labour Government and would do a disservice for workers, trade unionists, and the citizens of Wales".
Mike Hedges stated that his Swansea East Constituency Labour Party voted in opposition to the changes, even though he was absent from the meeting.
Backbencher supports Senedd's principle of gender balance and more members, but warns: "Having these very large constituencies will make it very difficult to be close voters over such large areas.
He shared the concerns of the three unions and said that the plans "mathematically make it harder" for Labour to get a government.
If the plans are approved at Saturday's special conference they would have to be sent to the Welsh Parliament as a new law. It will require the consent of two-thirds (or more) of the Senedd for the plan to become effective.