When Rep. Helen Bentley (R-Md.) was asked this week if she was surprised by Sen. Charles McC. Mathias' decision not to run for reelection in 1986, she first shook her head no, then paused and said: "Well, it surprised me more now then it would have two months ago."

"Recently I saw a lot of signs that he was going to run," Bentley said. "He's been more active."

Close Mathias staff aides, who also had been unaware of what Mathias would do, said they, too, had noticed that Mathias had been convening public meetings, plunging into legislative and political controversies, and generally shifting into high gear, campaign style.

"I was stunned," said one aide after an emotional half-hour staff meeting on Friday, during which Mathias announced his decision to retire.

Many Maryland Republicans were also stunned, because his decision not only set off a GOP scramble to fill the U.S. Senate seat but also sparked concern among GOP candidates who were hoping that Mathias would bring traditional Democratic votes to the ticket.

Bentley, for instance, said that during talks to many Republican groups over the past year she has stressed the value of having a popular vote-getter like Mathias at the top of the ticket next fall.

"I didn't want to lose him up there," Bentley said. "It disappointed me."

Bentley and other Republicans are quick to point out that the Maryland GOP has other possible candidates it considers strong. At the top of everyone's list seems to be former U.N. ambassador Jeane Kirkpatrick. But the Bethesda resident is an unknown quantity in Maryland electoral politics, and no one knows whether her celebrity status will translate to electability.

In addition, a source close to Kirkpatrick says the former ambassador is not interested in running, unless Maryland GOP officials beg her to do so, agree to raise funds and promise that she will have no primary opposition.

Meanwhile, some Republicans predict that conservatives will seek to solidify their power in the Republican Party with the departure of Mathias.

Moderates fear that this could prove devastating to the GOP in a state where only 24 percent of the registered voters are Republicans. They argue that only a moderate Republican, one who could lure large numbers of Democrats, can win statewide.

Unfortunately, say some Mathias supporters, there are few such Republicans around any more. And that, says one source, was probably why Mathias knew it was time to go.

Mathias appeared to be out of step with the national Republican Party, ignored and isolated by the conservative wing that swept to power with the Reagan administration.

He knew that unless the conservative domination waned, he would gain little additional power and influence in the Senate, say sources.

Meanwhile, Mathias would probably face his toughest challenge ever from a Democrat next year, requiring a year of grueling schedules and distasteful fund raising.

Sources say he began thinking that after a quarter of a century in Congress, it might be wiser to retire gracefully and move on to another challenge, such as writing or teaching.

Mathias had seen the shifting political tides cause painful, and sometimes humiliating, defeats for fellow moderate Republicans such as Sen. Clifford Case (R-N.J.), Sen. Jacob Javits (R-N.Y.), and Sen. Charles Percy (R-Ill.). They were men who also championed progressive causes: civil rights, arms control and protection of the environment.

With the ranks of the moderate Republicans dwindling, Mathias found himself snubbed by conservatives in the Senate at a time when his senior status and the new Republican majority in the Senate should have given him real power. Instead, he saw far more junior members -- such as Sen. Richard G. Lugar (R-Ind.), who became chairman of the Senate Foreign Relations Committee -- wield considerably more influence.

Yet Mathias loves the Senate -- the ceremony, the camaraderie, and the attention given senators, friends say. He is a brilliant man who is fascinated by the issues of world affairs, even giving up a seat on the politically powerful Appropriations Committee to take a position on the Foreign Affairs committee.

Most of his aides and friends believe he will dearly miss the Senate.

Mathias said of his decision: "I obviously thought about it for a long time, and kept the option of running open until within the last few days."

But, he added, "I don't think anything they could have done would have altered the decision."

Mathias said the time has come to pass the baton.

During his time on the Hill, "a new generation of Americans has grown to maturity, a generation whose quality I see when I look at our own sons," he said. "So I know that many able hands are ready to take up the work."