Sen. Lawton Chiles (D-Fla.), a low-key southerner who has been the Senate Democrats' chief strategist and spokesman on budget issues, jolted the Senate yesterday by announcing that he will challenge Sen. Robert C. Byrd (D-W.Va.) for minority leader.

His supporters said the secret-ballot vote next week is expected to be close.

Chiles' challenge to Byrd, who has run unopposed for party leader since his election to the post in 1977, apparently came in response to the election of new Republican leadership.

" Sen. Robert Dole R-Kan. is the leader of the Republicans and he's good," Chiles said. Republicans "voted for survival . . . . I don't think the Democrats are dumb. They're going to do the same thing."

This was a reference to the 1986 elections, when strategists for both parties agree the Democrats will have a good chance of reclaiming control of the Senate. Unless the Democratic Party takes advantage of that opportunity, Chiles said, it could be "doomed to be the minority party forever."

Immediately after Chiles' announcement, Byrd declared that he had enough votes lined up to win. "I have enough, I'm confident," he said at a news conference.

Some Senate Democrats have complained that Byrd's leadership has been lackluster and recently have worried privately that he would be no match for the new Republican high command. But despite rumors about secretive scouting expeditions on behalf of Chiles and one or two others, Byrd had been expected to be reelected without opposition at a party caucus next Wednesday.

Thus the Democratic leadership contest came as a surprise. Chiles said he decided only two days ago to make the race, which several of his colleagues had shied away from for various reasons, including fear of antagonizing the proud, touchy Byrd without having nailed down the votes to defeat him.

Chiles did not make any victory claims yesterday. Byrd's staff said Byrd had the committed support of more than 30 of the 47 Senate Democrats, including nine southerners.

But Chiles' supporters noted that the election is by secret ballot and that advance nose counts often prove faulty in Senate contests, as they did in last week's Republican leadership fight.

Asked if he was happy that the vote is to be by secret ballot, Chiles smiled and said, "The secret ballot is the American way."

Chiles, 54, came to the Senate in 1970 as "Walkin' Lawton," gaining national attention for traipsing across more than 1,000 miles of Florida to win a come-from-behind victory.

His career in the Senate was a low-profile one until early last year, when he became ranking Democrat on the Senate Budget Committee and point-man for the party in its budget battles with the Reagan administration.

Chiles' prominence increased dramatically this year, often eclipsing that of Byrd, as he led charges on issues ranging from budget deficits to the MX missile.

Despite Republican control of the Senate, a Chiles-drafted deficit-reduction plan failed on a tie vote. His push for a year-long moratorium on production of MX missiles also resulted in a tie vote, broken by Vice President Bush, and eventually contributed to congressional approval of a six-month delay in missile production.

Chiles also staged a quasi-filibuster on appropriations bills that ultimately forced a compromise to reduce the administration's defense budget for the current year.

Byrd's staying power has been formidable, however. He has catered assiduously to the needs of senators, often pushing them into the television limelight ahead of himself.

In his statement yesterday, he noted with pride his "record of service to my colleagues, to this institution and to the people of this country."

In response to critics who have said he doesn't project a forceful, telegenic image, Byrd said, "I don't think the party needs a superstar . . . . It needs someone who can forge a consensus . . . . I've been fair and even-handed, and Senate Democrats know that."

Saying the Democratic Party should move toward a more "centrist" position while also projecting itself as the champion of "change and new solutions," Chiles described himself as a "new face" who could help redirect the party.

"You can only circle the wagons so long," he said. "It comes time to hitch them up and move on."

It was unclear whether Chiles' challenge would stir up other contests. As of yesterday, party whip Alan Cranston (D-Calif.) and caucus secretary Daniel K. Inouye (D-Hawaii) were running for reelection without opposition. Sen. J. Bennett Johnston (D-La.) had earlier indicated interest in challenging Byrd's choice of Sen. George J. Mitchell (D-Maine) as chairman of the Democratic Senatorial Campaign Committee.