Stafford, of Aurora, said the decision was agonizing.
"Like many others in Colorado, I feel this way: I am not leaving the Republican Party as much as I believe the Republican Party left me," Stafford said.
Stafford becomes the 40th House Democrat, and the 20th woman in the caucus. Her move leaves the Republicans with 25 members, including four women.
Stafford voted against her former party last year on a bill that gave homebuyers more power to sue their builders for shoddy construction. At the time, she complained that Republicans and homebuilder lobbyists threatened her political future unless she voted against the bill.
The fight played in to her decision to switch parties, Stafford said Thursday.
She acknowledged that some constituents might be unhappy. Her ideal choice, she said, would have been to join a third party, but she knows third parties lack a strong voice.
"My answer was to join a party that better reflects my values and respects my contribution," Stafford said, prompting cheers from the two dozen House Democrats who joined her at the news conference.
Stafford is the first Colorado legislator to switch parties since 1987, when two Democrats joined the Republican Party, according to the Legislature's library. U.S. Sen. Ben Nighthorse Campbell of Ignacio created a stir in 1995 when he switched from Democrat to Republican in the middle of his six-year term.
Stafford did not tell GOP leaders in advance, but she did confer with Gov. Bill Ritter and House Speaker Andrew Romanoff, D-Denver.
House Minority Leader Mike May, R-Parker, was not available to speak, but he issued a written statement that wished Stafford well in her final year.
"I realize this is probably a significant personal moment for her, but politically her decision does not have much effect," May said. "This does not change the balance of power at the Capitol, and I believe that we will see a Republican in her place next year."
Stafford won her last election 60 percent to 40 percent, and registered Republicans outnumber Democrats 19,601 to 11,200 in her district, which also has 14,982 unaffiliated voters. Term limits will force Stafford to step down after the 2008 legislative session.
Rep. Ellen Roberts, R-Durango, was surprised at Stafford's move. As a moderate, Roberts is sometimes at odds with the majority of her caucus, but she has no plans to follow Stafford.
"I am a conscious Republican," Roberts said. "I believe in the Republican approach to government. I'm comfortable in my party, even in the minority status."
Roberts said she knows the GOP has to win back voters, but she wants to be involved in crafting solutions for the party.
"It's not for me to judge what Debbie Stafford's motivation is, and I respect her as a fellow legislator," Roberts said. "I'm only disappointed that she won't be in our caucus anymore."