"I believe our nation is at a crossroads," Griffiths said during a press conference in Alabama Tuesday. "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."
"For me to be true to my core beliefs and values, I must align myself with the Republican Party and speak out clearly on these issues," he added.
Griffith was one of 39 Democrats to vote against the party's health care reform bill last month. He is only one of three Democrats to have voted against the health care bill, the stimulus measure, and the cap-and-trade bill.
National Republican Congressional Committee Chairman Pete Sessions quickly welcomed Griffith to the Republican Party, and said Tuesday's events foreshadow big GOP gains in the 2010 elections.
"This decision is emblematic of the message that millions of concerned citizens have been trying to send to a Democrat Party that has become increasingly unwilling to listen," Sessions said in a statement. "Whether Democrat leaders choose to pay attention now or wait for the electorate to send them a resounding message next November, Parker Griffith's willingness to put the interests of his district first sends a positive signal to others who have grown just as disappointed with their party as the American people have."
Democratic Congressional Campaign Committee Chairman Chris Van Hollen expressed outrage over Griffith's decision and requested he return the money the DCCC invested to get him elected.
"House Democratic Members and the Democratic Congressional Campaign Committee took Parker Griffith at his word and, as a result, invested a great deal in working with Alabamans to bring Mr. Griffith to Congress," Van Hollen said in a statement. "Mr. Griffith, failing to honor our commitment to him, has a duty and responsibility to return to Democratic Members and the DCCC the financial resources that were invested in him."
Griffith won his seat in Alabama's 5th congressional district last year by 3 points over Republican Wayne Parker. But the district went for Republican John McCain over Barack Obama by 23 points in the presidential election.
Griffith's decision won't necessarily protect him from a GOP challenge in his conservative district. Shortly after news of the congressman's decision broke, conservative activists called for a primary challenger to take on the newly-minted Republican.
"Griffith was an extremely endangered Democrat," wrote RedState's Erick Erickson. "We should now hope him be an extremely endangered Republican in a primary. We will not fix the GOP's problems if we keep allowing people who are not one of us to suddenly switch the letter next to their name and magically become one of us."
With Griffith's switch, the Democrats will hold a 257-178 advantage in the House of Representatives.
- CNN's Deirdre Walsh, Evan Glass, and Paul Steinhauser contributed to this story