The evidence in weeks of hearings by the Senate Select Committee on Ethics raises significant questions about the conduct of three of the senators in the "Keating Five" case, special counsel Robert S. Bennett said yesterday.

In his two-hour closing argument, Bennett for the first time directly questioned the credibility of Sen. Donald W. Riegle Jr. (D-Mich.) and cited evidence that Sens. Alan Cranston (D-Calif.) and Dennis DeConcini (D-Ariz.) went beyond normal constituent service in aiding Charles H. Keating Jr., the operator of the troubled Lincoln Savings and Loan.

Bennett said the facts showed Sens. John McCain (R-Ariz.) and John Glenn (D-Ohio) -- the only one of the five who attended the session -- did nothing for Keating after the controversial meetings they attended with federal thrift regulators in 1987.

Keating raised more than $1.3 million for the senators' campaigns and causes. Attorneys for the five are scheduled to make their closing argument today, with Bennett then getting time for rebuttal.

Bennett said it would be a terrible mistake for committee members to conclude, as some of them have suggested, that the ethics case is unimportant in these critical times.

"Because with an economy in trouble and a war hanging over our heads, it is all the more important that the American people have confidence in their representatives, that they have confidence in the Senate, that they have confidence in you and your ability to judge your peers and reach an honorable and ethical decision," Bennett said.

"You are holding in your collective hands the heart, the political heart and soul of this country. And whatever decision you make . . . and on whatever basis you make it, the American people are either going to have more or less respect for this institution," he added.

Bennett argued, as he did in his opening statement Nov. 15, that the Senate has always used a set of ethical standards that includes one in which a reasonable person would see an "appearance of conflict." The five senators under investigation have repeatedly challenged that standard.

But Bennett cited several instances in which one of the five senators in the case refused to take some benefit from or action for Keating. He noted that Riegle returned more than $78,000 Keating raised for him because he was concerned about the appearance of the money being raised so close to the time of the April 1987 meetings with regulators.

The counsel also reminded committee members that the Keating case has captured the attention of the public. And the American people, he said, have asked "how is it Mr. Keating can get four U.S. senators" to one meeting and five to another.

Bennett raised the issue of Riegle's credibility in answer to a question by committee Chairman Howell T. Heflin (D-Ala.) about what difference it made whether Riegle helped set up the first meeting with regulators if he did nothing improper at the meetings.

Bennett said if the committee concluded there was "linkage" between Riegle's role in the meetings and a fund-raiser Keating sponsored for him a few weeks before, "that could turn something appropriate into something inappropriate." If there was no improper link, he said, "why is there so much substantial evidence of Senator Riegle obscuring his role."

He also said the committee had to consider whether Riegle was telling the truth when he testified. "If you were to determine that Senator Riegle was dishonest with you, I think this committee has to act on it."

In citing Cranston's solicitation of $850,000 from Keating for voter registration groups, Bennett said, "you could conclude there is overwhelming evidence" that on four occasions Cranston's appeals were made "knowing full well Charles Keating recently had asked for his assistance."

One such instance could be explained, he said. "But the committee must ask whether the second, third and fourth time, Senator Cranston should have known Charles Keating was trying to buy influence."

Bennett noted that DeConcini aggressively intervened for Keating in the meeting with regulators by "negotiating" for the executive. Bennett pointed out that McCain and Glenn thought negotiating was improper. He also said that DeConcini ignored several "red flags" about Keating's honesty and still made several calls to regulators urging they approve the sale of Keating's Lincoln Savings and Loan.