Sen. Paul S. Sarbanes (D-Md.), traveling his home state yesterday as he officially opened his reelection campaign, abandoned his normal reserve and excoriated New Right "extremist groups" that have targeted him for defeat.

Gathering force as he traveled from the front lawn of his boyhood home on the Eastern Shore to a packed Senate hearing room on Capitol Hill, Sarbanes berated the National Conservative Political Action Committee (NCPAC) and other "alien groups" for what he called campaigns of "distortion" and "manipulation.".

"We have an opportunity in 1982 to send a message not only across Maryland but across the nation that these groups should depart and depart now from our politics," Sarbanes told his audiences.

In a year-long media campaign NCPAC has spent $500,000 criticizing Sarbanes's voting record and suggesting it does not represent the views of most Marylanders. The attacks have focused national attention on the race and given Sarbanes a new aggressiveness as he prepares to face a Republican challenger in the November election.

"Six years ago he was a hum-drummer with speeches as dull as dishwater," said one fellow Democrat of Sarbanes. "Now he's a fighter, a Harry Truman candidate."

In his one term in the Senate and three in the House of Representatives, the 49-year-old Sarbanes has built an undisputed reputation as a down-the-line liberal and a brilliant thinker. But he also is viewed by some congressional aides and Maryland political figures as an overly cautious man, so private that he shuns the limelight even in the most public of professions. Such critics feel Sarbanes does not use his powerful office to help his state and influence the nation's affairs as much as he could.

"But it is tough to fault him on his role in the U.S. Senate," said one critic on the Hill who articulated this feeling. "He is there, he is faithful to the party and he contributes to the intellectual input. The thing is he isn't a leader, but then again, does every state have to have a leader?"

In an interview Sunday, Sarbanes was happy to answer that question. "I'm in my first term," he said. "I've been here five years. I like to get results and I feel you can get a lot of results often by letting others take the credit." He added that he has taken on "some important responsibilities and discharged them well," and said he believes he'll play more of a leadership role as a second-term senator.

Sarbanes' reputation as a brainy liberal is based largely on his role in the 1974 Nixon impeachment hearings in the House of Representatives, his leadership in the Senate to get the Panama Canal treaties ratified in 1978, and his more recent opposition to the Reagan tax and budget cuts. The liberal Americans for Democratic Action consistently has given him high marks for his voting record, including a 95 percent rating last year that placed him among the eight most liberal members of the Senate.

But for his committee work, the meat-and-potatoes of Senate life that voters seldom hear about, Sarbanes receives mixed reviews. One observer of the Senate Banking, Housing and Urban Affairs Committee calls him a "brilliant man with a creative mind" and a "lawyer's sense of going to the heart of a matter with the right question." Another describes Sarbanes as a cautious "me-tooer," who is typically aligned with the liberal viewpoint, but is seldom an energetic, active leader on issues -- a "very bright man," but one who "has trouble getting his questions out."

During his six-year term Sarbanes has introduced 16 pieces of legislation, a record now being attacked by at least one of the Republicans seeking to challenge him in November. By contrast, Maryland's senior senator, Republican Charles McC. Mathias, has sponsored nearly 240 bills and resolutions in the past four years.

Sarbanes said he has always believed that "popping in a lot of bills and issuing press releases is not the real substance of legislating." He said a senator can have more impact by helping to fashion major measures as they move through Senate committees.

On the Foreign Relations Committee, Sarbanes said he has worked to determine the amount of foreign aid sent to nations such as Israel and to press for nuclear arms control. On the banking committee, he said he helped shape major urban development programs and the 1977 legislation that blunted the effect of the Arab boycott of Israel by making it illegal for American businesses to comply with such boycotts.

Sarbanes' style has leaned to scholarly involvement in complex issues. Indeed, it was such an issue that helped propel the congressman from Baltimore into the Senate in 1976, the victor in battles with two better-known opponents.

Sarbanes built much of his campaign that year on his image as one of the white knights of the Watergate era, a role he played out before a television audience of millions during the 1974 House Judiciary Committee impeachment hearings. Indeed, it was the "Sarbanes article" of impeachment that was adopted by the committee in its first historic charge against Nixon, accusing him of covering up the Watergate scandal.

In 1978 Sarbanes again received national attention when he and Sen. Frank Church (D-Idaho) stood side by side defending President Carter's Panama Canal treaties during a historic 22-day debate. Conservatives attacked the treaties as a "giveaway" of the canal that would undermine America's strategic interests in Latin America. A coalition of liberals and moderates argued that the important thing was the canal's uninterrupted functioning and United States access to it, not its ownership. Both, they asserted, would be ensured by treaty ratification, which finally came by a one-vote margin.

During the debate, the meticulous, methodical Sarbanes allowed his more experienced colleague, Church, to make most of the speeches while he concentrated on the technical aspects of the treaties. In his customary way, Sarbanes immersed himself in the details of the agreements, studying them to decide how he would vote as a member of the Senate Foreign Relations Committee, on which he still serves. An aide remembers him carting a huge acccordian file filled "with every imaginable document" relating to the canal back and forth on his daily commute from Baltimore to determine his stand.

Today that stand in favor of the treaties has become a weapon in the hands of the New Right's NCPAC, which is using it in its anti-Sarbanes campaign.

Responding to Sarbanes' comments yesterday, NCPAC political director Vic Gresham said, "If NCPAC telling the truth about the senator's voting record does what he says it does, then I'm just sorry for the senator."

In fact, NCPAC's campaign has proved to be both a bane and a blessing to the junior senator from Maryland.

NCPAC officials say its polls show the campaign has cut deeply into Sarbanes' "favorable" rating among voters. But it also has galvanized his supporters and jarred the low-key politician into action, helping him build a bulging campaign warchest that he says holds about $800,000.

Around the country, a strong Sarbanes constituency, built through his voting record, is responding to the NCPAC challenge with checks to Citizens for Sarbanes. Labor unions, his traditional supporters, have contributed heavily through their political action committees. Money has come in from Jewish admirers, who say they appreciate Sarbanes' "friendship to Israel," proven by such actions as his vote last year against the sale of AWACS planes to Saudi Arabia.

He has cultivated the Greek-American community, a natural constituency that knows him as the first Greek-American elected to the Senate, by championing such causes as the embargo on the sale of U.S. arms to Turkey. Sarbanes was part of a band of pro-Greek legislators who pushed the embargo through Congress in 1975 after Turkey's 1974 invasion of Cyprus, a republic with a heavy Greek population. As a senator, Sarbanes remained part of this group of lawmakers, who became known as "the Gang of Four," and unsuccessfully sought to block President Carter's lifting of the embargo in 1978.

Now a member of the minority party in the Senate, Sarbanes is going to the voters these days during what he calls "some of the most difficult times this country has faced." He is attacking the Reagan administration for record high unemployment, programs that he says enrich the wealthy and "squeeze" the middle class and working people, and budget cuts that threaten education for the young and Social Security for the elderly.

"It's no secret, I have stood against much that the Reagan administration has done," he told his supporters yesterday in Salisbury in front of the white house where he grew up. "I'm against it . . . because I think it's the wrong vision for this country."