After 13 years with Mark Rutte as their prime minister, the Dutch will cast their ballots on Wednesday in a national election that is expected to scatter votes across the spectrum. But there is one man who has emerged as the campaign’s chief protagonist.

It is Pieter Omtzigt, a longtime parliamentarian and founder of a new party, who says he wants to overhaul the Dutch political system from the political center — appealing to voters increasingly disillusioned with the establishment yet wary of extremes.

Mr. Omtzigt, 49, has offered voters a novel mix of left-leaning economic policies and right-leaning migration policies, packaged in a party he created this summer, called New Social Contract.

“It’s a protest party in the political middle,” said Tom Louwerse, a political scientist at Leiden University who created a website that combines and summarizes polls.