Sen. Mark Dayton (D-Minn.) announced this afternoon that he will retire after just one term.

“I do not believe I am the best candidate to lead the Democratic-Farmer-Labor Party next year,” said Dayton.

Already under pressure from Republicans salivating at the prospect of knocking him off in 2006, Dayton had been gearing up for a tough re-election fight by raising his profile, hiring a speechwriter and telling the DSCC that he would rely heavily on the committee for financial help.

The department store heir spent $12 million of his personal fortune in his successful 2000 bid to unseat then-Sen. Rod Grams (R), but has said he could not afford to contribute nearly as much to his re-election effort. Dayton repeated that in his announcement today.

Dayton came under heavy fire in the fall when he closed his Washington, D.C., office, citing security concerns.

He was pilloried on Capitol Hill and in home state newspapers for his decision, which he said he made because he could not ask his staff to take the risk of remaining in D.C. while he was safely back in Minnesota.

Dayton remained defiant in public but had internalized much of the criticism directed at him in recent months, according to people close to him.

Dayton is the first Democratic Senator up for re-election in 2006 to announce his retirement. On the GOP side, Majority Leader Bill Frist (Tenn.) is keeping his self-imposed two-term limit pledge and will step down as well.

Paul Kane contributed to this report.