Democrats Wary as Two Senators Decide to Retire

WASHINGTON — The sudden decision by two senior Democratic senators to retire shook the party’s leaders on Wednesday and signaled that President Obama is facing a perilous political environment that could hold major implications for this year’s midterm elections and his own agenda.

The rapidly shifting climate, less than a year after Mr. Obama took office on the strength of a historic Democratic sweep, was brought into focus by the announcements that Senators Christopher J. Dodd of Connecticut and Byron L. Dorgan of North Dakota would retire rather than wage uphill fights for re-election.

With the chances growing that the election in November would end the 60-vote majority Democrats enjoy in the Senate — the practical threshold for being able to overcome united Republican opposition — the president and his party face additional urgency to make progress on his agenda this year.

There was no immediate sign that the developments would further complicate White House efforts to secure final passage of Mr. Obama’s main domestic priority, the overhaul of the health care system, but the political pressure on Democrats from competitive states and districts will not make it any easier.

Following on the heels of the news of the senators’ retirements, Gov. Bill Ritter Jr. of Colorado, a Democrat, also announced Wednesday that he would not seek a second term.

Together, the developments heightened a perception that a conservative push against the president’s ambitious agenda, a sluggish recovery from the deep recession and an outbreak of angry populism have combined to deplete Mr. Obama’s political strength and give Republicans a chance for big gains in this year’s races for the Senate and the House.

To the degree that the retirements reflect increasing skepticism among voters about the direction Democrats are pushing the country, Mr. Obama could face a tougher time winning legislative support as he presses ahead with initiatives on climate change, financial regulation, education and other issues.

Republicans seized on the resignations as a way to raise money and generate enthusiasm among voters in their conservative base.

“Voters and donors out in the country see two senior Democrats, both of whom were perceived to be safe a year ago, now retiring for fear of losing,” said Rob Jesmer, executive director of the National Republican Senatorial Committee. “This further underscores our belief that with each passing day, the environment is getting better and better for Republicans, which energizes our people and demoralizes theirs.”

The White House and Democratic Party leaders reached out on Wednesday to reassure other potentially vulnerable Democrats in an effort to prevent any more retirements or party-switching. Obama aides played down the developments, saying it would be foolish to make predictions now about the November elections before Mr. Obama had even delivered his State of the Union address. If health care legislation passes and the economy improves, advisers believe the president and the party will be in a stronger position by fall.