In July, Timmermans said he wanted to become the next prime minister and would be a candidate to lead the joint ticket.
Both parties now have until Aug. 22 to vote on his candidacy.
The elections follow the collapse of the four-party coalition government of outgoing Prime Minister Mark Rutte, over a failure to reach an agreement on restricting immigration.
If the parties confirm Timmermans as their lead candidate, he would need to leave his EU post.
A European Commission spokesperson said in July that Timmermans would not be required to immediately step down, since his candidacy "remains hypothetical" until the parties confirm him as their pick in late August.
(Reporting by Stephanie van den Berg; Editing by John Stonestreet)

European Commission Executive Vice-President for the European Green Deal Frans Timmermans (L) during a conversation with Limburg youths about climate adaptation in Valkenburg, the Netherlands, 16 July 2023. Severe storms caused flooding in parts of Germany, Belgium and the Netherlands in Limburg in 2021. EPA-EFE/MARCEL VAN HOORN