ASHLAND, Wis. — Representative David R. Obey has won 21 straight races, easily prevailing through wars and economic crises that have spanned presidencies from Nixon’s to Obama’s. Yet the discontent with Washington surging through politics is now threatening not only his seat but also Democratic control of Congress.

The fight for the midterm elections is not confined to traditional battlegrounds, where Republicans and Democrats often swap seats every few cycles. In the Senate, Democrats are struggling to hold on to, among others, seats once held by President Obama and Vice President Joseph R. Biden Jr. Democrats are preparing to lose as many as 30 House seats — including a wave of first-term members — and Republicans have expanded their sights to places where political challenges seldom develop.

“It’s not a lifetime appointment,” said Sean Duffy, a Republican district attorney here in the Northwoods of Wisconsin, where he has established himself as one of the most aggressive challengers to Mr. Obey since he went to Washington in 1969. “There are changes in this country going on, and people aren’t happy.”

Mr. Obey, who leads the powerful Appropriations Committee, is one of three House Democratic chairmen who have drawn serious opposition. Representatives John M. Spratt Jr. of South Carolina, who oversees the Budget Committee, and Ike Skelton of Missouri, who runs the Armed Services Committee, have been warned by party leaders to step up the intensity of their campaigns to help preserve the Democratic majority.

These established House Democrats find themselves in the same endangered straits as some of their newer colleagues, particularly those who were swept into office in 2008 by Mr. Obama as he scored victories in traditionally Republican states like Indiana and Virginia.

Representative Pete Sessions of Texas, chairman of the National Republican Congressional Committee, said he would consider anything short of taking back the House a failure. Republicans say they have not recruited strong candidates in all districts, but both parties agree that Republicans are within reach of capturing the 40 additional seats needed to win control. Republicans also are likely to eat into the Democratic majority in the Senate, though their prospects of taking control remain slim.

Democratic Congressional officials — well aware that a president’s party typically loses seats in midterm elections — have long been preparing for a tough year. But that Mr. Obey here in Wisconsin and other veteran lawmakers like Representative Earl Pomeroy of North Dakota suddenly find themselves in a fight reflects an increasingly sour mood toward the Democratic Party and incumbents.

“He’s supporting the party line of the Democrats, which is not consistent with North Dakota,” said Rick Berg, a Republican state representative from North Dakota who is challenging Mr. Pomeroy. “In the past, we’ve been more conservative at home than the people we send to Washington.”

Asked if this was a good time to be a Republican candidate, Mr. Berg laughed and said, “I sure think so.”

Mr. Pomeroy, who has served for 18 years as the state’s only congressman, won two years ago with 62 percent of the vote. Now he is among the top targets of House Republicans and is fighting without the help of one of the state’s incumbent Democratic senators on the ballot, since Byron L. Dorgan chose to retire.

“Some cycles are more challenging as a candidate than others,” Mr. Pomeroy said. “This should be in the range of challenging cycles.”

Democrats worry that some lawmakers who have avoided tough races in the past could be at added risk of defeat because they are out of practice, slow on their feet and often reluctant to acknowledge the threat they are facing. The chairman of the House re-election effort, Representative Chris Van Hollen of Maryland, has called mandatory face-to-face meetings with vulnerable members to monitor their campaigns.

In the Seventh District of Wisconsin, which covers 17,787 square miles from the middle of the state to Lake Superior, signs of Mr. Obey’s service in Congress are found in new bridges, highway expansions and countless other projects. Yet there are fewer signs of Mr. Obey himself. At the Democratic Party office in Wausau, his hometown, campaign placards hang in the window for Senator Russ Feingold, but none for Mr. Obey.

When asked to discuss his re-election bid, Mr. Obey declined, saying that it was too early to begin talking politics and that he was focused on his legislative duties. “I have never met anyone who thought political campaigns were too short,” he said.

Mr. Obey, 71, was elected two years before Mr. Duffy, 38, was born. Mr. Duffy is widely seen as leading in the Republican primary — his opponent is the candidate who lost to Mr. Obey two years ago by 22 percentage points — and his race has drawn support from party leaders in Washington, Tea Party activists and Sarah Palin.

He said he decided to challenge Mr. Obey because of his leading role in the economic stimulus bill, health care legislation and the growth of government. “I know that I can have a serious impact on the direction of the country if I could take out Obey,” he said.