Sen. Gary Hart (D-Colo.), who appeared to have his reelection script in good order only a month ago, has suddenly been thrust into the role of playing the heavy against a Rocky Mountain Joan of Arc.

As much as any region, perhaps more, the West has a history of embracing gritty, combative, odds-defying mavericks, especially with those with a flair for suspense and melodrama.

For the moment at least, Colorado has found just such a hero -- or more precisely heroine -- in Mary Estill Buchanan, who had to move mountains in the form of the state Republican establishment just to get on the party's senatorial primary ballot.

When she finally cleared all the legal hurdles and then defeated three other Republicans in the Sept. 9 primary, throwing the campaign plans of both parties into disarray, Buchanan captured the fancy of enough prospective voters to leap ahead of Hart in a late September poll by the Denver Post. The newspaper poll dovetailed with the results of an earlier television survey.

The Denver Post poll showed Buchanan leading Hart by 2 percentage points among all possible voters and by 15 points when probable nonvoters were winnowed out.

Hart disputes the findings as little more than a reflection of the suspense-filled publicity over Buchanan's primary victory and the three-month buildup to it. But he concedes that the race is now a lot closer than it was a month ago -- "four points either way," he says unenthusiastically.

"She was Joan of Arc, Aimee Semple MacPherson, this feisty little lady against all those big guys," sighed Colorado Democratic Chairman Mark Hogan the other day. "It's about the only thing that could have gotten Gary off track . . . the only question now is whether this 'gutsy little Lady' thing will wear off."

Whether it does or not, Buchanan's bell-ringing crash through the starting gate has turned the Colorado Senate contest into one of the country's most volatile races of the year -- once again an enticing target for Republicans in their bid to capture the Senate this year or two years hence.

Democrats -- and some neutral observers -- contend that Hart's appeal is solid enough to stay the course. But they are no longer as confident as they once were. "people are judging Gary on the issues and her on something else," said a Hart aide. "There are people who say, 'Sure I'll vote for Gary but I'm also going to vote for that neat lady.'"

From the start, the GOP targeted Colorado as one of its half-dozen best prospects for 1980, in part because the state has grown more conservative and Republican since 1974, when Hart, just two years after managing Sen. George McGovern's 1972 presidential campaign, was first elected to the Senate from here.

But the Hart-Buchanan contest is vastly different from the New Right vs. Old Left clashes in South Dakota and other Senate battleground states that have given an ideological sheen to the fabric of congressional politics this year.

Hart has had to contend with Republican charges that he's a liberal wolf in the sheep's clothing of a moderate and that his eclectic record spells opportunism rather than the thoughtful discrimination of an independent thinker. At the same time, Buchanan has gotten off to a shaky start in contending with major campaign issues, pointing up Hart's relative expertise on the substance of legislation.

Nonetheless, both candidates generally come across as bright, attractive, highly educated and serious mainstream politicians -- earnest debaters of economic, defense and energy issues who tend to leave their audiences better informed, if somewhat less wakeful, than they found them.

Hart, 42, a Kansas-born former divinity student and Yale Law School graduate, moved early in his Senate career to blur old stereotype and stake out an image of a pragmatic, post-New Deal, future-oriented Democrat.

He championed a leaner but stronger military machine, became a budget-cutter and simultaneously pushed for energy development and environmental protection -- all the while tending carefully to Colorado water projects, the state's resource development potential and other local interests.

He has supported such controversial Carter administration policies as the congressionally blocked oil import fee (as an energy conservation measure) but has avoided a close identification with the administration, which is not popular here, and cites his support of Carter only when asked.

Buchanan, 45, a San Francisco native who graduated from Wellesley College and Harvard Business School before becoming a management consultant, has been more popular with the voters than with the dominant conservative wing of her own party. She has twice been elected to the largely record-keeping office of Colorado secretary of state, most recently by a landslide in 1978.

In addition to taking a moderate position on state issues, and sometimes cooperating with Democratic Gov. Richard Lamm on tax and other matters, she has supported the Equal Rights Amendment and opposed antiabortion measures, thereby failing critical right-wing litmus tests.

When she failed to get enough support at the state GOP convention in June to win a place on the September primary ballot, Buchanan petitioned her way onto the ballot over party leaders' objections and fought off an attempt to block her candidacy in court that was finally rejected by the Colorado Supreme Court only five days before the primary.

In the primary, she defeated the presumed frontrunner, former secretary of the Army Howard (Bo) Calloway, by less than 1 percentage point.

Although Colorado Republican leaders claim that the party is now unifying behind her and she has moved to embrace conservative national GOP policies, there are still signs of stress.

Paul Weyrich, director of the Committee for Survival of a Free Congress and a leader of the New Right Coalition, was quoted by Denver papers recently as saying he'd just as soon have Hart win over Buchanan. "We worked very hard to defeat men like [Republican Sens.] Jacob Javits [of New York] and Clifford Case [of New Jersey], so why would we support another just like them?" asked Weyrich rhetorically.

A greater problem for Buchanan appears to be her sometimes embarrassing lack of familiarity with national issues that Hart has mastered, which could come to be critical in the issue-oriented, high-tone campaign that has evolved between the two.

When Buchanan ran a full-page newspaper ad criticizing 37 Hart votes on defense issues but failed to say how she would have voted on many of the same issues, the Rocky Mountain News (which is backing Hart) ran a big headline reading, "Buchanan Ducks Defense Vote Issue."

At a joint appearance before the Denver Bar Association last Monday, she attempted to fault Hart on defense spending by referring critically to a Hart fund-raising letter describing him as "one of the strongest arms control advocates" in the Senate -- a blurring of issues that raised some eyebrows in the room full of lawyers.

But she has also landed some blows, calling Hart's record "confusing" and assailing his earlier votes to trim defense spending as having "set the pace for the military decline we have been living with ever since." She drew appreciative laughs from the bar association crowd in twitting Hart for voting against eliminating 62 patronage-placed elevator operators from the automatic elevators in the Capitol.

Colorado Republican Chairman Philip Winn contends that Buchanan's strength stems largely from a growing perception of Hart as a liberal and a Carter administration supporter, an impression fostered in part, he said, by GOP literature dumped on 200,000 doorsteps the day after the primary election.

Hart's organization and financing are rated as superior to Buchanan's, although she is getting fund-raising help from Senate Minority Leader Howard Baker (R-Tenn.) and other party luminaries in an attempt to match Hart's $750,000 target for campaign spending. For his part, Hart contends that the four Republican candidates together will have outspent him by 2 to 1 or more.

Buchanan, a career woman who has raised six children, competes strongly for the women's vote and for the votes of independent moderates who could well decide the election. According to the Denver Post poll, Buchanan is also running ahead of Hart among older (but not young) voters and in urban (but not rural) areas.

Buchanan has also made it more difficult for Hart to fight back with an aggressive counterattack because it might well prompt another wave of sympathy for her.

"She does have an ability to capitalize on persecution or presumed persecution," complained Hart in claiming he would confine himself to protecting his record against "distortions." But he has also prepared new media advertising that, according to aides, stresses his "experience and ability" in an indirect way of suggesting her lack of it.

Hogan, for one, contends that Buchanan's surge in the polls could help Hart in the end: "When I tried to raise a few bucks for Gary [before the poll results came out], the reaction was, 'Oh, hell he doesn't need any help.' Well, they've been disabused of that notion now."