Jesse Helms only a year ago was regarded as potentially one of the most powerful members of the Senate, possessor of one of the sharpest swords in town.

The conventional wisdom had him about to lead a charge of conservative New Right knights across the political landscape, cutting a swath through established government policies on prayer in public schools, abortion, school busing and other so-called conservative "social issues."

Helms had already built a formidable political organization around these issues, and proved himself an effective obstructionist against liberal legislation in the Senate. "But now the conservative current is swift, and Helms' time has come," Time magazine wrote in a cover story one year ago last week.

Since that time, the North Carolina Republican's standing in the Senate has fallen markedly. The qualities that serve him as outsider have not translated well, have not also made him an effective insider and committee chairman in the Republican-controlled body.

Helms' leadership of the Agriculture Committee, which he chairs, has come under fire; some critics say he has endangered the tobacco price support program upon which his tobacco-growing state depends so heavily.

He has been accused of high-handed tactics, and tying up the Senate. His credibility and methods have come under question from fellow senators. Though he may win a victory this week on school prayer, he lost in the Senate last week on abortion, and not a single proposal of his on social issues has become law.

"For all his huffing and puffing the emperor doesn't have any clothes," William Hamilton, a lobbyist for the Planned Parenthood Federation of America, declared last week after Helms' anti-abortion bill was beaten.

"Jesse has worn about as thin as tissue paper with most members," says one southern senator.

"It's not any one single thing," says the senator, a conservative Democrat. "It's not abortion. It's not school prayer. It's his tactics. It's what he did on tobacco. It's his fight with Tom Eagleton. It's his Congressional Club and how he makes you vote on emotional issues, and then sends a bunch of money into your state and tries to beat you."

"There are a lot of people laying in wait for Jesse, and not all of them are Democrats," says one congressional aide.

Helms brushes aside such talk and any suggestion that he is losing legislative battles to liberal foes.

"All I know is there was a fella around here named McGovern and a fella named Birch Bayh and a fella named John Culver and they beat Jesse Helms time and time, and now they're saying, 'I'm not in the Senate and Jesse Helms is,' " Helms said the other day. "So we'll see who wins and loses in the end."

Helms' reputation also remains intact among conservative New Right and anti-abortion groups. "He's absolutely still the generalissimo of our movement," says Peter Gemma, executive director of the National Pro-Life Political Action Committee. "He's gotten us what we wanted: recorded votes."

This continues to make Helms a man to be feared in the Reagan White House. Helms knows this, and he has used his position on the Senate Foreign Relations Committee to tie up the appointment of one State Department official after another on the grounds that they are not conservative enough. "There's a feeling," said one senator, "that Jesse has more clout downtown than up here."

But Helms' problems in the Senate haven't gone unnoticed back home in North Carolina. The Raleigh News & Observer, where Helms once worked as a sports reporter, recently accused him in an editorial of "squandering his own, as well as his state's, credibility in the Senate."

"In the legislative process, tactics and personal relationships count," wrote the newspaper. "It is difficult to see how Helms has helped himself with his colleagues by the way he has gone about pushing his social legislation."

The latest controversy around Helms involves his handling of the anti-abortion legislation. Among other things, he is accused of having reneged on an intricate agreement whereby three competing anti-abortion proposals could be brought to the floor. He denies he reneged, saying he was misunderstood.

But others say his account is untrue, and the incident damaged him in the eyes of peers. This was compounded by other Helms actions later. "People started to question his word, his integrity. That can be devastating around here," says one participant. " Sen. Bob Packwood (R-Ore.), a filibuster leader , wouldn't believe anything Helms said after that."

The Senate, during a three-week period, took five votes on the abortion issue. Helms lost them all, by margins which some congressional sources described partly as "personality votes" against Helms.

Helms' reputation as a legislator already had been tarnished by his leadership of the Agriculture Committee. Last fall, in Helms' first big test as a legislative quarterback, Sen. Robert J. Dole (R-Kan.) had to take over floor management of the farm bill to save it from defeat.

Helms angered Sen. Thomas F. Eagleton (D-Mo.) and other Agriculture Committee senators last June when he tried, without notice, to push a controversial tobacco price-support bill through his committee and onto the floor within hours.

Helms' reputation among Senate peers may not be so important. His power base is outside. It is, as Ronald Reagan's once was, among conservatives across the country. Now that Reagan is president, Helms is their most beloved spokesman.

During the last two years, he has raised $13 million from these conservatives through his Congressional Club. He has used this money to expand his influence by donating to other candidates.

There are some in the Senate who feel that Helms is less interested in passing legislation than in raising money for his and other conservative groups. Even if he loses, Helms raises emotions among his followers every time he mounts a legislative campaign, they say. He wins, in effect, because the next batch of fund-raising letters brings in more money to perpetuate his influence.

"You could almost hear the printing presses start running when the abortion debate began," says a Senate aide. "You could see the letters going out and the money coming back in."