WASHINGTON — Former Attorney General Eric Holder on Thursday formally announced a new effort aimed at challenging the partisan gerrymandering that’s left Democrats struggling to win local and state offices.

The new organization, the National Democratic Redistricting Committee, will devote legal and political resources to redrawing state districting maps — a process that will happen after the 2020 census. His group will have the strong backing of a powerful ally: President Barack Obama.

Over the next three years, Democrats will wage a state-by-state effort aimed at capturing key offices, waging legal challenges and winning ballot initiatives that affect how political maps are drawn.

“Those who control state government draw the lines that shape Congress for the next decade,” Holder said at an event hosted by the Center for American Progress, a liberal think tank. “Our goal is to make sure Democrats are in position to ensure fair and representative electoral districts.”

The Republican stronghold on power in Washington after the November elections has sparked a resurgence of Democratic interest in state and local elections. Holder’s organization aims to marshal the resources of the former president, liberal advocacy groups, the Democratic Governors Association and the Democratic Legislative Campaign Committee in gaining the power to recraft often creatively drawn districts in a manner that benefits their party.

Obama has said that redistricting will be a top political priority in his post-presidency, admitting that he failed to create a “sustaining organization” around the political coalition that twice elected him.