Shifting the Overton window involves proponents of policies outside the window persuading the public to expand the window.

How exactly does voting for a politician who supports roughly zero policies outside the Overton window persuade the public to expand the window?

Edit:

Although the window offers a theory of change, its central element—the window itself—actually describes the norm from which reality has deviated. Zeynep Tufekci worries in The New York Times that Trump “voices truths outside the Overton Window,” while the British writer Sam Leith speculates that Corbyn may have positioned his party “dangerously far from the centre of the Overton Window.” The window serves as shorthand for the erstwhile consensus. Viewing politics through the Overton Window reinforces liberal notions about the moderate center, even as that center ground erodes.

For conservatives, by contrast, the Overton Window has always been about strategy. Though Overton himself never committed his most influential idea to paper, his Mackinac Center colleague Joe Lehman continued his work after Overton’s death in 2003 at age 43. Lehman not only coined the term “Overton Window,” he weaponized it, setting up training sessions on the concept for other right-leaning think tankers. The term filtered into the conservative blogosphere in 2006, when Josh Trevino enthused about the window as a tool for the right. “Step by step, ideas that were once radical or unthinkable—homeschooling, tuition tax credits, and vouchers—have moved into normal public discourse,” Trevino declared. “The conscious decision to shift the Overton Window is yielding its results.”

Okay maybe don’t derive your argument from yet another piece of “libertarian” think tank nonsense.