But Democrats have long suffered from a consuming obsession with the presidency at the expense of other elections, said Mr. McAuliffe, a former national party chairman.

“We’ve got to be smarter about how we’re building the future of this party. We have been decimated at the state level, and it’s at the state level that they draw the maps,” he said, adding: “We raise all this money in the presidential, and then everybody goes away.”

The next round of congressional redistricting is still years away, after the next decennial census in 2020. But the officials drawing the maps in most states will be chosen in elections well before then, starting with the election for governor in Virginia this year.

Democrats ruefully acknowledge now that before the 2010 census, riding high after Mr. Obama’s 2008 victory and seemingly secure in their hold on Congress, they were far less prepared than Republicans in gearing up for legislative reapportionment. The Republican Party mounted a ferocious state-by-state campaign that gave it overwhelming control of redistricting, allowing it to lock in many victories in the 2010 midterm elections.

In the run-up to 2020, Democrats say, their goal in many places will be not to seize control of redistricting, but merely to capture one or two key offices to keep Republicans from locking them out of the process. Right now, there are 25 states in which Republicans control the whole government — the governorship and the legislature — versus just six wholly controlled by Democrats.

Mr. Holder said his initiative would unfold on three fronts: In court, where Democrats will challenge Republican-drawn maps they see as violating the law; on the campaign trail, where they will seek to win offices that influence redistricting; and through ballot referendums in states that allow voters to give direct approval to laws mandating new procedures for legislative apportionment.

Mr. Holder said he was also prepared to take an unaccustomed leap into electoral politics, campaigning for candidates around the country who can affect the redistricting process. A career prosecutor, Mr. Holder has quickly emerged as a leading figure in Democratic efforts to fight Mr. Trump; he has been retained by the Democratic-led California Legislature to help in any battles with the Trump administration.