EVERGREEN, Colo. (AP) _ Gary Hart announced today he would not seek a third term in the U.S. Senate, but kept mum about any aspirations he has for pursuing the Democratic presidential nomination in 1988.

Addressing a noontime gathering of about 300 supporters in this mountain community west of Denver, the 49-year-old Colorado Democrat broke a months- long silence about his political plans, saying, ''In 1987, I will not be representing Colorado in the U.S. Senate, but I will continue to be a voice for Colorado.''

Colorado's senior senator moved to the forefront of possible contenders for the Democratic nomination after Sen. Edward Kennedy announced he would not seek the presidency in 1988. Other Democrats considered in the running are Gov. Mario Cuomo of New York, Sen. Joseph Biden of Delaware, and Rep. Richard Gephardt of Missouri.

In 1984, Hart's darkhorse candidacy nearly derailed Walter Mondale's quest for the Democratic nomination. Hart won the New Hampshire primary and placed second in the Iowa caucuses to become the No. 1 challenger to Mondale.

Hart proclaimed himself the ''new-ideas'' candidate and advocated ''a new generation'' of leadership.

He urged reforming the Pentagon, overhauling the Medicare system, and a budget combining defense cuts, tax hikes and $20 billion in new spending each year for education, nutrition, job training, housing, roads, bridges and sewers.

While Hart's campaign ultimately failed, it thrust the Colorado senator into national prominence and set the stage for another possible attempt at the presidency in 1988.

The 1984 campaign also left Hart with a campaign debt of more than $4 million, of which $3.5 million remains.

Colorado Republican Party Chairman Howard ''Bo'' Callaway said he understands why Hart would want to start running for president early.

''It makes sense for him with his debt and the close election last time and the positions he has taken with Ronald Reagan,'' said Callaway.

The GOP chairman said Hart knows he could be beaten in a Senate re-election try. ''We would look forward with great relish to running against him (in a Senate race) and winning,'' Callaway said.

Hart's career in the Senate began in 1974 when he upset Sen. Peter Dominick by linking him in voters' minds with Watergate. He barely won re-election in 1980 over former Colorado Secretary of State Mary Estill Buchanan.

A recent poll showed Hart would get 50 percent of the vote in a Senate race this year. The poll also found Coloradans would be less likely to vote for Hart if they felt he would abandon his seat in two years to seek the White House. He has been criticized for his poor Senate attendance record the past two years.

Most political observers had assumed Hart would retire from the Senate this year, opening up that seat to a campaign by Hart's friend, Rep. Tim Wirth.

Three Republicans already have begun campaigning for Hart's seat: Rep. Ken Kramer, state Sen. Martha Ezzard and Terry Considine, Callaway's son-in-law.