U.S. Rep. Parker Griffith spoke to reporters at his home in northern Alabama, a region that relies heavily on defense and aerospace jobs.
"I believe our nation is at a crossroads and I can no longer align myself with a party that continues to pursue legislation that is bad for our country, hurts our economy, and drives us further and further into debt," Griffith said as his wife Virginia stood by his side.
The 67-year-old radiation oncologist was narrowly elected last year in a district that includes Huntsville and Decatur. President Barack Obama lost badly there to Republican John McCain.
Griffith also slammed the health care overhaul making its way through Congress. He was one of 39 House Democrats to vote against a version of the bill that narrowly passed.
"I want to make it perfectly clear that this bill is bad for our doctors," he said. "It's bad for our patients. It's bad for the young men and women who are considering going into the health care field."
He said after the press conference that his defection had nothing to do with concerns about whether he could win re-election as a Democrat. He also said he had not talked to any fellow Democrats about switching parties along with him.
"If they do, I hope it's on conviction and not politics," he said.
Alabama Democrats defended Griffith against GOP claims that he was soft on terrorism during the 2008 election, and the head of the state party said he is disappointed by Griffith's defection now.
"Democrats of every stripe and philosophy sweated and bled for this man," said Joe Turnham, chairman of the state party. "He narrowly became a congressman through the hard work, votes and financial contributions of thousands of Democrats. Today, they feel betrayed."
Turnham said Griffith should return money to Democratic donors - something the congressman said he would be happy to do.
Griffith had accumulated one of the most conservative voting records of any House Democrat. He was one of seven Democrats to oppose Obama's economic stimulus measure early this year and also voted against an anti-global warming bill pushed strongly by House Speaker Nancy Pelosi, D-Calif.
Democrats will hold 257 House seats to the GOP's 178 after Griffith's switch.
Several veteran House moderates have announced their retirements next year, giving Republicans hopes of picking up a significant number of seats in the November elections.
Jim Spearman, executive director of the Alabama Democratic Party, said the switch "shouldn't come as a surprise" with the way Griffith voted.
"We will be working strongly to put a Democrat in there," Spearman said.
Associated Press writers Charles Babington, Frederic J. Frommer, Sam Hananel and Andrew Taylor in Washington contributed to this report.