WASHINGTON (AP) _ Rep. Bill Grant, a little-known Democratic congressman from a conservative district in the Florida Panhandle, bolted to the Republican Party on Tuesday in a move President Bush hailed as ''good news for our party.''

Grant easily won election twice as a Democrat, even as his state was moving toward Republican candidates.

''Florida is on the move,'' Bush, who met with Grant about the switch last week, said before network television cameras. ''The nation, I think, benefits from this.''

House Speaker Jim Wright, leader of House Democrats, said:

''It took me by surprise, but that's everyone's choice. I hope he's happy.''

Grant, 46, a former banker from the small town of Madison in the state's Panhandle, was officially changing his voter registration later in the day.

His switch tips the balance in the makeup of Florida's House delegation, leaving it at 10 Republicans and 9 Democrats and breaking the Democratic Party's long-time majority.

Bush said the change ''is good news for our party, not only in Florida, not only in the South, but nationally.''

Democratic National Chairman Ron Brown said ''I'm frankly not real concerned about it.'' But some other Democrats voiced outrage that Grant would change parties so soon after the election, and Rep. Beryl Anthony, D-Ark., chairman of the Democratic Congressional Campaign Committee, said finding a Democratic challenger would be a top priority for the committee.

Charles Whitehead, Florida state Democratic chairman, called Grant ''a political opportunist'' who plans to run for statewide office. He said Grant should resign and seek his seat again as a Republican.

Whitehead said Grant supported Democratic presidential nominee Michael Dukakis last year and, after appearing at a joint campaign event, ''emphatically requested that he be allowed to have photographs made with Governor Dukakis.''

Bush was asked if Grant's change helps the party overcome its embarrassment at the election Saturday of David Duke, a former leader of the Ku Klux Klan, to a seat in the Louisiana Legislature as a Republican.

Bush defended his stated opposition to Duke, and that of Lee Atwater, the national GOP chairman, but acknowledged that ''maybe there was some feeling in Metairie, La., that the president of the United States involving himself in a state legislative election was improper or overkill.''

Grant met with the president Thursday about changing party affiliation and made his decision over the weekend, said Republican consultant Charles Black.

Black, who knew Grant through mutual political friends, put Grant in contact with Atwater, who arranged the meeting with the president.

Grant said his switch ''is not going to change my values, nor will it change the way that I vote.''

''After a period of prolonged and careful deliberation, I have determined that I can better represent the values and the priorities of north Floridians in the State of Florida as a Republican,'' he said.

Another Florida congressman, Andy Ireland, switched from the Democratic to Republican party four years ago.

The most successful of the recent party switchers has been Sen. Phil Gramm of Texas, who became a Republican in 1982 as a House member and later won election to the Senate. Unlike the later Florida defectors, however, Gramm resigned his seat when he made the switch and then won election to his old seat in the special election that followed.

Grant served four years in Florida's state Senate before winning election to the House in 1986, when 12-term Rep. Don Fuqua, a Democrat, retired. Grant won 51 percent of the vote in a five-candidate primary and had no Republican opponent in the general election.

Grant's 2nd congressional district stretches along the border with Georgia from near Panama City to just west of Jacksonville and includes Tallahassee, the state capital.

Last fall, two Republican upsets in races for traditionally Democratic congressional seats eroded the Democrats' majority in the Florida delegation. At the same time, Republicans picked up one of Florida's Senate seats with the retirement of Democrat Lawton Chiles.

Grant has been active in the Democratic Party and had figured in speculation for a possible run for governor in 1990. He drew publicity last summer for his unsuccessful campaign in support of Sen. Bob Graham for the Democratic vice presidential nomination.

Rep. Guy Vander Jagt, chairman of the National Republican Congressional Committee, called Grant's switch ''further proof that we are moving toward majority status.''

Grant's decision hardly changes the overall House balance, however; Democrats have a 258-175 advantage, with two vacancies, after the switch.