The union endorsed the former emergency measures minister Thursday, boosting his campaign to become the province's next premier by 62 delegates.
The NDP is currently electing about 2,200 delegates across the province who will vote for the party's next leader at a convention in March.
Unions are automatically awarded a block of delegates but may not be able to fill them all with qualified candidates under the rules.
An unofficial count of elected delegates so far puts Ashton in front, following closely by former cabinet minister Theresa Oswald.
Premier Greg Selinger is trailing his competitors so far but has the support of CUPE, which is allotted 288 delegates.