Washington (CNN) - Democrats reeling from last week's upset loss of Ted Kennedy's U.S. Senate seat in Massachusetts suffered a new setback in Arkansas on Monday as a longtime Democratic House incumbent announced his intention to retire at the end of this year.
The party's hopes also dimmed in Delaware, where popular Democratic state Attorney General Beau Biden declared that he will not seek his father's former U.S. Senate seat this November.
Joe Biden vacated the seat after he was elected vice president in 2008.
The twin announcements increased speculation that the Democrats may suffer major congressional losses in the looming midterm elections.
Seven-term Rep. Marion Berry, D-Arkansas, cited health concerns in his announcement that he won't seek re-election this fall.
"The people deserve a representative who has the ability to rise to the numerous challenges that face our state and our nation," he said in a written statement.
"As a lifelong farmer, time has taken its toll on my health and I am no longer able to serve the district with the vitality I once possessed. ... I am at perfect peace with this decision and look forward to returning to the farm and my home state of Arkansas."
Berry, a member of the fiscally conservative Democratic "Blue Dog" coalition, represents a district covering the northeast portion of Arkansas. While he was unopposed for re-election in 2008, GOP presidential nominee Sen. John McCain beat then-Sen. Barack Obama in the district by 21 points.
Four years earlier, President George W. Bush defeated Sen. John Kerry by 5 points in Berry's district.
Berry becomes the sixth House Democrat over the past two months to announce his retirement. The others are fellow Arkansas Democrat Vic Snyder, Reps. Bart Gordon and John Tanner of Tennessee, Rep. Dennis Moore of Kansas, and Rep. Brian Baird of Washington.
Rep. Robert Wexler, D-Florida, stepped down at the beginning of the month. A special election is scheduled for April to fill the vacancy.
Democratic leaders are now keeping a close eye on their colleagues, hoping to persuade any other Democrats contemplating retirement to instead run for re-election in November.
Adding to House Democrats' woes: another six members of their caucus are running for statewide office instead of seeking re-election.
Alabama Rep. Parker Griffith switched from the Democratic Party to the GOP at the beginning of the year.
Only three GOP congressmen, in contrast, are retiring at the end of the year, with 11 others running for statewide office.
The Democrats currently hold a 256-178 advantage in the House, with Wexler's seat accounting for the one vacancy. Republicans need a 40-seat pickup in November's midterm elections to win back control of the chamber.
There is currently only one Republican competing for Berry's seat, but others are expected to enter the race in the wake of Berry's announcement.
The announcement came on the same day that Beau Biden announced his intention not to run for the seat held by his father for 36 years.
"I cannot and will not run for the United States Senate in 2010," Biden said in an e-mail to supporters Monday. "I will run for re-election as attorney general."
Biden's decision gives Republicans a strong chance to pick up the seat. Their likely candidate, Mike Castle, is Delaware's lone congressman and a former two-term governor of the state.
In his statement, Biden mentioned the prosecution of a high-profile child molestation case as a reason he won't run for the Senate.
"My first responsibilities are here in Delaware," he said. "The immediate need (is) to focus on a case of great consequence. And that is what I must do."
The vice president's old seat seat is currently held by former Biden aide Ted Kaufman, who was named as an interim replacement for the vice president after he resigned from the Senate.
Kaufman announced Monday that he will not seek a full term.
A CNN/Opinion Research Corporation poll released Monday indicates that Americans are divided on whether Democratic control of Congress is good for the country.
Forty-five percent of people questioned in the survey said Democratic control of Congress is a good thing, with 48 percent disagreeing. The margin is within the poll's sampling error.
The results are a shift from last June, when 50 percent felt that Democratic control of both chambers of Congress was good and 41 percent felt it was bad for the country.
–CNN's Paul Steinhauser, Mark Preston, and Alan Silverleib contributed to this report