Sen. Gary Hart (D-Colo.) said today that he will not run for reelection this year, a move that will permit him to focus his political energies on the 1988 presidential campaign.

"It's time for me to express my commitment to our state and our nation in other ways, and perhaps on a further horizon," Hart said.

"Does that mean I'm making some announcement about '88? Nope. Does it mean I still have an interest in being president? Yup."

Hart's decision not to seek a third term means Rep. Timothy E. Wirth (D-Colo.) almost certainly will be the Democratic nominee for the seat, which both parties consider pivotal to the contest for control of the Senate in the next Congress.

There are three candidates for the Republican Senate nomination. Any one of them would be a serious contender in this state, which is trending toward Republican registration and voting patterns.

Hart, after putting off for months a decision about his 1986 plans, turned his announcement of non-candidacy into a major political event.

On a sparkling winter afternoon, hundreds of Colorado Democrats and a swarming battalion of reporters, camera crews and others jammed a foothills restaurant just below the snowy peaks of the Continental Divide. The guests snacked on venison and buffalo as they awaited Hart's brief explanation.

A key consideration for Hart has been the impact of a third Colorado race on his 1988 presidential hopes.

Some advisers have told Hart that a run for reelection would be too costly -- he still has $3.4 million in debts from his 1984 presidential race -- and too risky for a person with presidential ambitions. Hart won by just 1 percentage point in his last race, and national conservative groups had targeted him for defeat this year.

But others urged Hart to take the gamble and run, on the theory that a 1986 victory here in prime Reagan country would give him a leg up on other Democratic presidential hopefuls in 1988.

Hart said today that "it was tempting" to run again for the Senate and "defeat the far right wing." But he said Wirth, who has no opposition for the Democratic Senate nomination, "will get the job done."

Hart said he will "be a voice" on national issues "to focus the attention of our nation on our unmet agenda for the future." He cited unemployment, farm problems, defense spending waste and "the injustice of Reaganomics" as issues he will deal with after leaving the Senate.

Hart's withdrawal from the Senate race probably won't diminish Democratic chances of holding his seat. If Hart had run for reelection, the race would have been a toss-up. As of today, it seems equally a tossup with Wirth carrying the Democratic banner.

Wirth was one of the "Watergate babies" first elected to Congress in 1974, and he has maintained a generally liberal voting record since.

Republicans here have tried to label him an ultraliberal, an ugly moniker in a state that gave Ronald Reagan 63 percent of the vote in 1984. But Wirth has used his position on the Energy and Commerce Committee to build an alliance with the state's business community, undermining somewhat the charge that he is excessively liberal.

Wirth has been a leader in Congress on telecommunications issues, and Republicans here say they think that gives them a potent issue: They blame Wirth for the breakup of the Bell telephone system. Wirth angrily denies this charge. But the issue proved effective for the GOP in 1984, when Wirth had a close contest for reelection to his House seat, and surely will come up in the Senate race.

The best-known and thus the leading contender for the Republican Senate nomination is Rep. Ken Kramer (R-Colo.), a strong conservative whose four terms in Congress have been marked by his forceful advocacy of the Strategic Defense Initiative or "Star Wars" antimissile defense system; Kramer was for it before Reagan signed on.

Kramer has been touring the state and planning this Senate race for four years. But the talk among Colorado Republicans is that his campaign is poorly organized. His fund-raising to date has fallen well short of the amounts raised by House members running for the Senate this year in other states.

Another conservative GOP contender is businessman and party activist Terry Considine, a millionaire who is ready to spend freely on the race.

Considine is well-connected: He is the son-in-law of the state Republican chairman and has been endorsed by Sen. William L. Armstrong (R-Colo.), the state's most popular Republican. But Considine has never run for office and has minute name recognition outside a small cadre of party workers.

The moderate on the Republican side is state Sen. Martha Ezzard, a veteran of several battles with her party's dominant conservative wing. Many Democrats say they think Ezzard would be the strongest challenger to Wirth because of her appeal to unaffiliated voters and women of all parties. But she may not be sufficiently conservative to win a Republican primary here.

Because the seat Hart is vacating is so clearly up for grabs, the race has been "targeted" by numerous interest groups for special funding and attention.

A poll of 400 voters last month by The Denver Post showed that most of those surveyed are not familiar with any of the candidates. It showed Wirth and Kramer running even, but Wirth well ahead of the two other Republican contenders.