Minnesota Sen. Mark Dayton (search), a first-term Democrat atop the Republicans' 2006 target list, said Wednesday he won't run for re-election next year.

"I do not believe that I am the best candidate to lead the party to victory next year," Dayton told reporters on a conference call. "I cannot stand to do the constant fund raising necessary to wage a successful campaign, and I cannot be an effective senator while also being a nearly full-time candidate."

His decision is likely to open the way toward an expensive open-seat election battle in a state that has become more receptive to Republicans in recent years.

Republicans control the Senate, 55-44, with one Democratic-leaning independent. Apart from Dayton, Senate Majority Leader Bill Frist, R-Tenn., has long signaled his intention to retire at the end of his term in 2006, although he has not recently confirmed plans to retire.

"Everything I've worked for, and everything I believe in, depends upon this Senate seat remaining in the Democratic caucus in 2007," Dayton said.

Dayton, 58, was elected to his seat in 2000, defeating conservative Sen. Rod Grams.

The victory was his first in electoral politics, coming after years spent in public life in Minnesota's liberal Democratic politics. Dayton served as an aide to former Sen. Walter F. Mondale (search), and was state auditor for four years in the 1990s.

Republicans had made little secret of the fact that they intended to run hard at Dayton next year. GOP Rep. Mark Kennedy (search) was among those considering a challenge.

Heir to a family department store fortune, Dayton spent about $12 million of his own money in the 2000 race but said he wouldn't do it again.

He struggled to raise money for a re-election campaign, and parted ways in the past few months with two top financial aides he had brought on to help jump-start his fund raising.

Dayton came to the Senate offering himself as a liberal alternative to the conservative Grams, and he often demonstrated a plainspoken style to go with it.

At a recent confirmation hearing for Secretary of State Condoleezza Rice (search), he accused her and other Bush administration officials of "lying to Congress, lying to our committees, lying to the American people" in the run-up to the war in Iraq.

He also drew headlines last fall when he closed his Washington office, citing a top-secret intelligence report that he said made him fear for his staff's safety.

Federal law enforcement officials said at the time there was no new intelligence information indicating the Capitol complex was a terrorist target. Republicans ridiculed Dayton, arguing his decision sent a dangerous signal that Americans would give in to terrorist threats.