His stern, square face stretched into a wide, satisfied grin, Sen. Gary Hart (D-Colo.) gazed out over an imaginary audience and recited a line he has been polishing for months.

"Apparently my Republican challenger doesn't think Colorado is important enough to have a candidate for president," Hart intoned, clearly loving every syllable. "Well, I don't think New York and California are the only states entitled to have presidential candidates -- I think Colorado is just as important as any other state."

The setting is not a meeting hall, but a kir-and-quiche restaurant in basic Hart country, a Yuppie Denver neighborhood where the "new-ideas" Democrat is rehearsing that line Just in Case -- just in case he decides to run for a third Senate term next year, just in case an opponent charges that Hart would sacrifice Senate duties to his burning presidential ambition.

But it is a line he may never use.

After months of agonizing, Hart said he is still weeks away from deciding whether to run for the Senate again in 1986 or step aside and concentrate on the 1988 presidential campaign.

As he promised after his second-place finish in the 1984 race for the Democratic presidential nomination, Hart, 48, is running hard for the White House. But there is 1986 to be solved.

Until this summer, conventional wisdom in both parties held that Hart would not seek reelection to his Senate seat.

For one thing, politicians and pundits agreed, a 1986 campaign probably would cost about $2 million. Raising that sum would interrupt Hart's effort to pay off his 1984 campaign debt -- he is more than $2 million short -- and to build a treasury for 1988. For another, there is a good chance that Hart could lose a Senate race here -- a potentially fatal blow to his presidential chances.

In his last Senate campaign, Hart won by 1 percentage point over a relatively weak opponent, scoring with a final splurge of spending fueled by some $250,000 in contributions from political action committees. But in light of his attacks on PACs in 1984, Hart says, he probably will not take contributions from such groups again.

And this time some solid GOP challengers -- led by conservative U.S. Rep. Ken Kramer and moderate state Sen. Martha Ezzard -- are lining up for the Senate race.

For all those reasons, most politicians here had expected Hart to step aside in 1986 so that his liberal compatriot, Rep. Timothy E. Wirth (D-Colo.), could run for the Senate seat. Wirth, whose Boulder district is becoming increasingly Republican, is itching to run statewide.

But in recent weeks, a new Democratic poll and Hart's assertions of complete confidence -- "If I run, I'll definitely win," he said -- have prompted new speculation.

A poll commissioned by state Democratic Chairman Buie Seawell showed Hart with a 63 percent approval rating, roughly his average during 11 years in office, and a hard-core negative rating (from those who say "I'll never vote for Hart") of 19 percent. Other polls here had shown Hart's negative rating almost twice as high.

"The Republicans think they can beat me up for [the presidential race] in 1984," Hart said. "But the negative fallout on that isn't much. People in this state are proud that a Coloradan won half the Democratic primaries in the country."

Hart also brushes aside criticism of his 37 percent Senate attendance record during that campaign year. "People don't expect me to fly back to vote on every National Pickle Week resolution," he snorted.

On this point, state Republican Chairman Howard (Bo) Calloway agrees. Although he is confident that the Republicans could unseat Hart, he said, "we're not going to win by hitting him for 1984."

Meanwhile, some Hart advisers have suggested that a 1986 reelection campaign could boost the candidate's 1988 prospects. If Hart were to win in a western state that gave President Reagan 63 percent of its vote in 1984, he could present himself to Democrats as a candidate who can win in Reagan country.

Although Hart said he is attracted by that gamble, he argued that he has proved the point.

"I won in Colorado in 1980, when Reagan was on the ticket and I had Jimmy Carter's name above mine," he said. "I've shown I can win. If anything, it's the new guys like [Arizona Gov. Bruce] Babbitt who have something to prove."

Hart offers withering assessments of "new guys" in the 1988 Democratic presidential contest -- including Babbitt, Rep. Richard A. Gephardt (Mo.) and Sen. Joseph R. Biden Jr. (Del.) -- who are basing their campaigns, Hart said, on ideas he championed in 1984.

"Now, Bruce Babbitt is being set up as a 'Gary Hart with passion,' " Hart said in a voice thick with sarcasm. "I know Bruce, and if there's one thing he hasn't got, it's passion. Gephardt is supposed to be 'the new Gary Hart.' Biden is coming on as 'Gary Hart with brains,' or something. Don't they think people are going to want the real Gary Hart?"

All of that leaves Hart in disagreement with those who say he shouldn't run for reelection here in 1986, and in equal disagreement with those who say he should.

"Let's just say I'm thinking carefully about it," he said. "And I probably won't decide until November, because that's when I've promised to announce what I'm going to do."