What electoral reform instituted in the 19th century and supported by Progressives led to less party-centered elections?

the adoption of the secret ballot

Explanation:

Following the Presidential election of 1884, the adoption of the secret ballot came into effect in the 19th century in the United States starting with Massachusetts.

Before this adoption, the election period or finance is characterized by the financial muscle of the party and they influence how the voters tend to vote, through vote-buying, harassment of voters, and even direct assault.

However, with the adoption of the secret ballots, their party-centered elections got reduced as the states become responsible for providing the ballot for the voters rather than the voters themselves or the party.