First-term Democratic U.S. Sen. Mark Dayton of Minnesota, who polls indicate is vulnerable to defeat, announced on Wednesday he would not seek reelection next year.

“I do not believe that I am the best candidate to lead the … party to victory next year,” Dayton, 58, said in a conference call with reporters in which he took no questions.

Dayton, a department store heir, reached into his own fortune to help finance his 2000 campaign when he ousted Republican Rod Grams, who may again run for the seat.

A party source said Dayton was having difficulty raising money and was also troubled by recent poll numbers that showed his approval rating below 50 percent.

“I cannot stand to do the constant fund-raising to wage a successful campaign, and I cannot be an effective senator while also being a nearly full-time candidate,” he said.

“Thus,” Dayton said, “I choose to devote all my time and energy to the job Minnesotans elected me to do” in the Senate. His term ends in January 2007.

Republicans control the Senate with 55 of the 100 seats and have made Dayton a top target in the 2006 elections. Democratic presidential candidate John Kerry carried Minnesota in his failed bid last year to unseat President Bush.

Thirty-three of the 100 Senate seats are up for election next year, 15 of them now held by Republicans, 17 by Democrats and one by a Democratic-leaning independent.

Jennifer Duffy, who tracks Senate races for the nonpartisan Cook Political Report, said she was not surprised by Dayton’s decision. “I think without a doubt Dayton was the most vulnerable incumbent senator up for reelection,” she said.

A number of Republicans are seen as potential candidates for Dayton’s seat, including U.S. Reps. Mark Kennedy and Gil Gutknecht as well as former Republican Sen. Grams.

Several Democrats were expected to consider running for the seat, though there appeared to be no immediate front-runner.