A gang of street kids from Portland's tough munjoy Hill neighborhood is roaming the city in a Winnebago ready to descend on command to hit the target of the day - a shopping center parking lot or a block of row houses.

Its weapons are campaign leaflets and its ultimate leader is Republican Rep. William Cohen, who is widely expected to succeed in his effort to unseat Democratic Sen. William Hathaway in a race that has drawn high-level interest of both national parties.

Cohen is one of the Republicans' best bets to pick up a Senate seat in an election year that promises them slim pickings nationwide, and President Carter has headed a long list of Democrats flying into Maine in attempts to save Hathaway.

In addition to providing visitors, the White House has changed its position on the land claims of Maine Indians' in an effort to resolve one of the issues most damaging to Hathaway.

The White House agreed this week to what Maine state officials - and Cohen - have demanded, offering for this first time to have the federal government bear all costs of a settlement with the Indians.

Hathaway is a liberal in a state where liberalism's drummer has never dominated and in a year when Maine's voters - like many others - are angry at taxes and big government.

Hathaway, 54, has tried to make a virtue of being somewhat out of step with Maine's voters. "I think, by and large, people trust someone who doesn't agree with them all the time," the Senator said this week.

"You know where he stands," is a Hathaway slogan aimed at capitalizing on the senator's unchallenged reputation for honesty and implying that Cohen is, as one Maine Democrat said, "the perfect media man, all style and no substance."

According to Maine polls, however, the voters prefer someonw who does agree with them and Cohen is on the popular side of every major issue. If Cohen was perceived as a liberal when he entered Congress in 1972, the perception was in error, Cohen said. "I've always been a conservative on fiscal matters."

Government is too big, he tells audiences. "Never in our history have the people been so disenchanted or alienated by their institutions."

Cohen's rhetoric undoubtedly has won him points, but his superior campaign organizations is going to win him as many votes. For Hathaway, an upset winner six years ago, history seems to be repeating itself - except he is cast in the other role this time.

Hathaway had youth, an appeal for change and a better campaign working for him when he beat Margaret Chase Smith in 1972. Smith hardly campaigned while her underdog opponent traveled the state.

Now, Hathaway has been out campaiging hard, but not as hard or as long as Cohen. He is 16 years older than Cohen, and it is not hard to find voters who grumble that they haven't seen Hathaway around Maine since his last campaign.

That the young boys from traditionally Democratic Munjoy Hill have been working for Cohen rather than hanging out on street corners after school is only one example of Cohen's local-level campaign.

Other Democrats have been urging Hathaway to let them do more on his behalf and have warned him of cohen's inroads in Democratic areas, but the Hathaway campaign has not responded quickly.

Against Cohen's local organization, Hathaway's flow of Democratic visitors has included Sen. Edward M. Kennedy (D-Mass.), Secretary of Health Education and Welfare Joseph A. Califano, Jr. Secretary Jr. Secretary of Agriculture Bob Bergland, Vice president Mondale, and four Carters - the President, his wife, his mother and his son Chip.

The president makes his second visit next Saturday, and there is speculation here that he will bring another gift for Hathaway: Loring Air Force Base in northern Maine has been designated to be shut, but the president could take it off the hit list.

Since Maine's 665,000 registered voters are divided almost in thirds between Republicans, Democrats and independents, no one can win with his party's support alone. Hathaway proved his appeal in 1972, but Cohen has carried several Democratic towns in his last two congressional campaigns and is an excellent campaigner.

"Cohen's the most attractive candidate this state has had since the young (Sen.) Edmund Muskie," a state educator volunteered.

Muskie, who was governor from 1954 to 1958 and then went to the Senate, revived Maine's Democratic Party, which had become a small club. This year, following the trend Muskie began virtually singlehandedly, registered Republicans for the first time.

It is a trend Cohen would like to reverse, and if he wins he will become the dominant Republican in the state.

Cohen said he is campaigning for the long haul, and he has taken time even in Democratic areas to make joint appearances with local GOP candidates.

College friends from Bowdoin said Cohen chose to become a Republican because he saw the Democratic Party crowded with people his age.

Now, Cohen is attracting new, young candidates into the Republican fold, and the party hopes to come out from under the shadow of Muskie's domination and the four-year term of Gov. James Longley, an enormously popular independent. Longley is stepping down after one term, much to the relief of Republicans and Democrats.

Cohen first received national attention in 1974 as a Republican member of the House Judiciary Committee who voted for the impeachment of President Richard M. Nixon.

It is a moment he doesn't let voters forget. "Bill Cohen was tested the hard way. He did Maine proud," one Cohen television commercial says.

Democrats ask what tests Cohen has taken since then. He has not authored significant legislation and Democrats call him a show horse who has spent his time politicking while Hathawa has been a workhorse in Washington.

Cohen has raised a Maine record of $480,000 for his campaign, almost $230,000 of it in out-of-state contributions that include many from large corporations.

Hathaway has raised $318,000, much of it from labor unions. About $140,000 of Hathaway's contributions are from outside Maine.

Three independents are challenging Hathaway and Cohen, but only one of them, former state senator Hayes Gahagan, was expected to attract many votes.

The campaign of the 30-year-old conservative has foundered, however, since he announced that someone had implanted the word "sex" on his face in his campaign photographs.

He says he has since discovered the same word appears in Cohen and Hathaway campaign photographs and he makes no claim to know who is doing the implants. But he is calling for a congressional investigation of what he terms "a national scandal" of subliminal advertising.

His problem is that people outside his campaign, including this reporter, can't see the words even with the aid of a magnifying glass.