"AS MAINE GOES, no goes the nation." That old nostrum of American political wisdom provides as good a reason as any to keep a close watch on this year's Maine Senate race. In a year when most election contests are referenda on the impact of President Ronald Reagan's economic policies, the candidates in Maine offer voters perhaps the clearest choice anywhere between "staying the course" with Reaganomics and giving the Democrats another chance.

Maine's race pits incumbent Democrat George Mitchell, who was chosen by Gov. Joseph E. Brennan to replace newly appointed Secretary of State Edmund S. Muskie in 1980, against challenger David Emery, a Republican congressman from the first district in eastern Maine. Emery, a young outspoken conservative, "makes no bones about his support for Reagan," says campaign spokesman Bob Tyrer.

Like the President, Emery favors more defense spending, wants a constitutional amendment mandating a federal balanced budget, and opposes any move to cancel the third year of supply-side tax cuts Emery also backed the President's $98 billion "mid-course correction" tax increase.

Mitchell, a soft-spoken liberal who has come back from a sizable early deficit in the polls, voted against the tax increase and against the balanced budget amendment, which he labeled "a farce." The former judge and U.S. attorney--who lost his only previous bid for office in the 1974 gubernatorial campaign--wants to reduce the federal deficit by deterring the third year of supply side tax cuts Emery also backed the president's $98-billion "mid course correction" tax increase.

Despite these sharp differences, however, the voters remain evenly divided, with each side claiming the "momentum" and a narrow edge in the polls Mitchell, appealing to Maine's many poor and to the unemployed in hard-pressed industries like textiles, shoes, and lumber, has been hitting hard at Emery's support for what Mitchell spokesman Steve Morrison calls "Reagan's trickle down approach" Emery says he's a "mainstreamer" on economics, and is pinning his hopes on Maine's relatively few 78 percent overall unemployment rate and on the conservative Republican instincts of the state's many small farmers. Both sides agree that the outcome could hinge on Emery's ability to hold on to traditionally Democratic voters in mill towns like Lewiston. Portland and Westburg--voters who have backed him for the House, but who now fear that their jobs may be the next to go.

THAT THE RACE is still too close to call may actually prove that the voters like Emery-Reagan economic because almost every other issue in the campaign has worked in favor of Mitchell The acid rain problem is a case in point Maine's tourist economy is heavily dependent upon sport fishing, and the poisoning of many of the state takes by acid rain has driven away both the fish and the out of staters who came to catch them Mitchell scored points when he addressed this concern by sponsoring an acid rain bill and pushing it through the environment and public works committee. The Democrat like to point out that Emery didn't co sponsor the House version of the bill until seven months after it was introduced. And Mitchell can boast endorsements from virtually every major environmental organization in the country as a result of his efforts.

Mitchell also seems more in step with Maine public opinion on the issue of a nuclear weapons freeze, which he ardently supports Maine's legislature, following the example set by 81 town meetings in the state, endorsed the freeze by voice vote last year. Moreover, a recent informal survey by the Maine Sunday Telegram found that more than 70 percent of the people in the state favor a freeze. Emery, on the other hand, is the only member of Congress from New England opposing a freeze.

Slip-ups by Emery's own campaign have also asked Mitchell in his comeback bid. An early Emery statewide campaign mailing claimed that Mitchell was soft on defense and offered as proof an alleged rating of zero given Mitchell by the Veterans of Foreign Wars. But the VFW made that rating when Mitchell had only been in the Senate for two months and had not yet voted on any major defense legislation. Emery was forced to make an embarrassing retraction, and later fired his campaign manager and press secretary.

The toss-up in Maine doesn't bode well to Democratic hopes of a big anti-Reagan turnaround in today's voting. It ominous indeed that a solid moderate like Mitchell with a popular record on arms control and the environment--not to mention a volunteer force of 3,000 strong--isn't pulling away from a staunch Reaganite like Emery, who hasn't even run a very effective campaign. Mitchell has an even shot at pulling it out, but Democratic effort to capitalize on the country's economic travail could very well fail to Maine, where many people seem still to cling to the hopes of prosperity that Ronald Reagan stirred in 1980. And as Maine goes.