When Sen. Robert T. Stafford (R-Vt.) appeared to be in trouble in his primary race last month, the new political arm of the environmental movement flexed its muscle.

The League of Conservation Voters bought radio spots in every major Vermont town, featuring Hollywood's most glamorous environmentalist, Robert Redford, extolling Stafford's fights for the Clean Air Act and toxic waste cleanup. The Sierra Club deployed volunteers to man his phone banks. Friends of the Earth organized a get-out-the-vote drive aimed at pro-Stafford precincts. Several environmental groups raised about $20,000 at fund-raisers in New York and Washington.

When Stafford won, he issued a public thank-you to the environmental movement and posed for pictures with one arm around a leader of the Sierra Club, the other around a staffer for Friends of the Earth.

It was a heady moment for environmentalists, who in 1982 are making their first big plunge into national electoral politics.

Stafford's victory helped establish the "green vote" as a force to be reckoned with. It proved that the movement could produce money and field workers to turn out votes for its friends in the clutch. It also masked some of the obstacles environmentalists face in this election year. While national polls show dissatisfaction with the Reagan administration's environmental record, the issue appears greatly overshadowed in voters' minds by the slumping economy and other concerns.

In a recent Washington Post-ABC News poll, 66 percent of the voters rated the environment "very important" -- a relatively high score, but well behind such issues as the threat of war (89 percent), unemployment (87), inflation (86), tax equity (86), crime (84) and protecting Social Security (81).

However, this has done little to discourage environmental activists, who have flooded into selected campaigns across the country, providing volunteers and money for the gritty but essential grass-roots tasks of canvassing precincts, stuffing envelopes and getting voters to the polls. The movement now has five national political action committees compared to 1980 when only one, the League of Conservation Voters, took part in a congressional campaign. Together, they have endorsed more than 150 House and Senate candidates, raised about $1.7 million and mobilized thousands of volunteers.

"After the 1980 election, we were faced with a Congress that was unlobbyable. We had to change the players to get the kind of legislation we need and that every poll shows the people want," said Bob Chlopak of the new Friends of the Earth PAC.

Partly because of their sheer numbers in the field, environmentalists have wedged their issue into campaign debates across the country. When they endorsed Democratic Sen. George J. Mitchell of Maine for reelection, his challenger, Rep. David F. Emery responded by claiming than he had done more than the incumbent to fight acid rain.

In Florida, the debate turns on clean water and beach development. In the Rocky Mountains, it is the Reagan administration proposal to sell public lands. On the West coast, it is Interior Secretary James G. Watt's accelerated offshore drilling plan.

The groups have taken up some of the tactics used by the New Right in 1980, such as direct-mail fund-raising letters that capitalize on the negative image of their opponents. Putting Watt's name in a mailing works as well for environmentalists as Sen. Edward M. Kennedy's (D-Mass.) does for the New Right, according to several direct mail consultants.

The Sierra Club has computerized the 1.1 million names from its 1981 petition drive seeking Watt's ouster, supplying them to candidates endorsed by the club's PAC. Rep. Les AuCoin (D-Ore.) raised $6,000 in his first mailing using the list, and plans a second.

The groups have spent most of their time and money on behalf of environmentalists in Congress who face serious threats in 1982. Stafford became a high priority as chairman of the Senate Environment and Public Works Committee.

A similar rescue mission has been mounted for Rep. Phillip Burton (D-Calif.), a longtime leader of legislative battles for clean air, clean water, park and wilderness protection. Last weekend, an estimated 150 environmentalists walked door to door for Burton, who faces liberal Republican state legislator Milton Marks.

Like their counterparts in the New Right, the PACs have also compiled an unofficial "hit list," targeting congressmen who scored lowest on an environmental voting index compiled by the League of Conservation Voters. For example, the league has mounted a special campaign to aid the opponents of seven Republicans on the House Interior Committee who, a league mailing claims, "act just like clones of James Watt."

But in conservative Utah, environmentalists are keeping their heads low, mainly working phone banks, as they and several Democratic groups pool resources in a longshot bid to unseat Republican Sen. Orrin G. Hatch. In this part of the country, many view the environmental movement as a threat to jobs.

Although the environmental PACs portray themselves as nonpartisan, their favored candidates are virtually all Democrats. Stafford is one of the few exceptions, along with Sen. John H. Chafee and Rep. Claudine Schneider, both of Rhode Island, and a handful of others.

Even that has created some internal strains. Vermont environmentalists are bucking the national organizations to support Stafford's Democratic challenger, who they claim has even stronger credentials.

Polls show that voters across the country believe Democrats do a better job than Republicans in protecting the environment. No other issue is so clearly "owned" by Democrats except job opportunities for women, according to a Democratic National Committee strategist.

Republican Party operatives say they do not view this as a threat. "Everyone is for the environment, but is that the issue that is going to get people to the polls? Our surveys show the answer is no," said Vince Breglio, pollster for the Republican Senatorial Campaign Committee.

One Democratic strategist concurs. "Frankly, if somebody gave me a million dollars for advertising, I wouldn't spend it on the environment. I'd spend it all on Social Security. We're counting on environmentalists in the field to make it a voting issue."