WASHINGTON — Russ Feingold, the former Democratic senator from Wisconsin, is assuming the leadership of the American Constitution Society, a progressive group active on judicial nominations and the justice system, signaling that Democrats are planning an aggressive effort to sharpen their focus on the federal courts as a defining issue.

Mr. Feingold’s elevation is the latest sign that Democrats are looking for ways to counter President Trump’s efforts to reshape the federal judiciary through a three-year campaign — steered by Senator Mitch McConnell, the majority leader — to confirm conservative judges for lifetime appointments. It comes as Mr. Trump has renewed criticism of judges he claims are biased against him, while seeking to impose his own will on the justice system.

Mr. Feingold, best known as a crusader for overhauling campaign finance laws, said he believed the Republican push to stack the federal courts with conservatives, along with mounting questions about political interference at the Justice Department, has led to a crisis moment that called for a heightened response.

The former senator, who taught at several leading law schools after losing his seat in 2010, said he intended to reinvigorate the organization and intensify its efforts to counter what he saw as an assault on the justice system.