“Democrats have done a poor job, and I take my share of responsibility here, in not being as focused as Republicans have on building at the grass roots,” Mr. Axelrod said. “Look what the G.O.P. and their related agents have done with legislative and City Council and school board races. They are building capacity, and Democrats have paid the cost.”

Many promising young Democrats in the House have been frustrated by the reluctance of Representative Nancy Pelosi, the minority leader, and her aging deputies to step aside and let new members ascend to leadership — one of the few rewards for a minority party in the House. “I was on the recruitment committee, and a lot of candidates decided to take a pass,” said Representative Karen Bass, Democrat of California. She added, “There are people who are new to Congress and have a difficult situation because they are not going to be there for 20 years.”

Some simply leave. “I was one of the few Democrats not to support Nancy Pelosi for leader,” said Representative Gwen Graham, Democrat of Florida, who is retiring after one term and planning to run for governor. “We need new voices.” Representative Chris Van Hollen of Maryland, once considered a potential House speaker, is running for the Senate.

Democratic ranks have also been decimated in state governments across the nation, where new leaders tend to plant roots for future higher office.

After the 2008 elections, Democrats controlled 62 of the 99 state legislatures; today, Republicans control 68 chambers, according to Governing magazine. Over the same time period, the number of Democrats in governor’s mansions fell from 28 to 18. In both cases, Republican control is now at or near historic highs.

Democrats point out that they have strong leads in Senate races in Illinois, Indiana and Wisconsin, and that most analysts believe the party is in a strong position to retake the chamber. They also note that many of their candidates are raising more money than their Republican rivals.