But in a sign of their fundamental advantages, Republicans have a far shorter list of races that concern them, and multiple party strategists said they believed there were only 35 to 40 Republican lawmakers in seats that Democrats could seize. In some of the most populous states in the country — including Michigan, Pennsylvania and Texas — Republicans believe they can lose no more than two or three seats, and perhaps fewer.

Former Representative Thomas M. Reynolds, Republican of New York, who led the National Republican Congressional Committee in 2006, said the congressional map was far better for Republicans now than it was a dozen years ago, the last time a Democratic wave election tipped control of the House.

“Redistricting has strengthened both state chambers and the Congress, and that is much more significant in 2017 than it was in 2006,” Mr. Reynolds said. “Look how many seats are in play. It’s not that many.”

In addition to gerrymandering, Mr. Holder and other Democrats have warned insistently about the impact of state-level voting procedures that threaten to disadvantage lower-income and minority voters. The Democratic Congressional Campaign Committee has begun to track states and localities where Republicans have total control of election administration, and Democratic aides say they plan to assemble lawyers around the country to monitor election machinery for “red flags” of partisan interference.

Yet the party’s ability to roll back the most significant voting restrictions may be limited. While federal courts have invalidated or reined in some of the most stringent restrictions — in North Carolina, Texas and Wisconsin — there are 17 states that now require voters to show photo identification at the ballot box. The types of acceptable identification vary, but Democratic-leaning minorities and young people are least likely to possess them.

The impact of those restrictions is difficult to measure, but experts say they do deter would-be voters from casting ballots. In Texas, where Democrats are targeting four House seats, Stephen Ansolabehere, a political scientist at Harvard University, analyzed the state’s voter rolls for plaintiffs in a federal lawsuit and concluded that 608,000 of the 13.5 million registrants lacked any of the seven IDs accepted at polling places. Black voters were two to three times as likely as whites to have none of the identifications; Hispanics one and a half to two times as likely.