The Iran-contra hearings resumed yesterday amid broad agreement in Congress that the investigation already has inflicted serious political damage to President Reagan and is steadily eroding his ability to function effectively in the remaining months of his term.

Interviews with House and Senate members from both parties who do not serve on the investigating committees disclosed a seeming paradox in the public reaction to the hearings, now in their seventh week: While many are impatient with the pace and the quality of information being produced, most sense a day-by-day erosion in the president's power and authority.

With words like "boring" and "a yawn," members of Congress said the hearings for the most part have not excited or engaged the attention of their constituents. So far there have been no explosive disclosures about Reagan's role in the affair, the so-called "smoking gun" that would cut through the welter of often confusing details surrounding the transactions, officials said.

"A lot of people are saying, 'Ho-hum, is this all?' " said Sen. Lawton Chiles (D-Fla.).

At the same time, however, the revelations of those details -- about shredded documents, private profiteering and the shadowy world of arms merchants and private foreign-policy operatives -- have taken a steady and inexorable political toll on the president and his administration.

"It's bleeding him," said Rep. Vin Weber (R-Minn.).

"That daily erosion is going on," added Rep. Guy Vander Jagt (R-Mich.).

Sen. George J. Mitchell (D-Maine), a member of the Senate investigating committee, complained recently that the White House has artfully succeeded in focusing attention "very narrowly" on whether Reagan knew of the diversion of funds from the Iran arms sales to the Nicaraguan rebels, thereby obscuring the "broader issues such as the rule of law and the wisdom of the policy."

But interviews with Mitchell's colleagues who are not directly involved in the investigation suggested that such underlying issues are filtering through the daily droning of testimony in the House and Senate committee hearing rooms.

Even while awaiting the testimony of the two key figures in the investigation -- Rear Adm. John M. Poindexter, the former national security adviser, and Lt. Col. Oliver L. North, the fired National Security Council aide -- congressional Democrats have concluded that Reagan and his administration have suffered severe damage.

Rep. Charles W. Stenholm (D-Tex.), a conservative supporter of Reagan's policy in Central America, said his constituents "would like to see us take the hearings off television," a common theme on Capitol Hill, where there is a clear sense of impatience with the length and pace of the investigation, particularly among Republicans.

Nonetheless, Stenholm said, the hearings have "damaged {Reagan} substantially" even among those who still support aid to the contras. The most damaging aspects, according to several lawmakers, have been the arms sales to Iran, which are just now being explored in detail, the suggestions of private profiteering and shifting administration explanations of the degree of Reagan's involvement.

"People are depressed, they're angry, that what they thought was one thing turned out to be another," said Sen. Bill Bradley (D-N.J.). "You hear it from Republicans and Democrats; they feel let down and depressed."

Some GOP lawmakers have reached equally harsh judgments about the political impact. "It's looked on as shoddy, sloppy, sordid," said Sen. John H. Chafee (R-R.I.).

Other Republicans point out the lack of a public outcry and stress that the hearings so far have not added startling new dimensions to the broad outlines of the Iran-contra affair that have been known for months. But even these lawmakers are quick to add that they recognize the seriousness of the ongoing investigation.

"I tell my Republican colleagues the fact that people are not asking us about it does not mean that we haven't suffered a good deal of damage, both to the president and to us as a party," Weber said.

The costs to Reagan already have become evident in a number of ways, beginning with what Sen. Wyche Fowler Jr. (D-Ga.) called "a betrayal of trust."

"People are programmed to be disappointed by politicians, but they thought he was different," Fowler said. "Their questions convey a betrayal of trust. It's not anger; it's far more a sense of sadness and loss."

Other lawmakers say the disclosures have produced a far more severe public reaction than sadness.

"On this issue, Reagan doesn't have enough credibility in Montana to fill a thimble," said Rep. Pat Williams (D-Mont.). "People tend to believe he is lying. I tend to think he is bewildered, confused and forgetful, but my constituents don't. They think he's lying."

The net effect, according to several lawmakers, has been a clear erosion of Reagan's political authority and ability to influence the course of events in the remainder of his term. Some of this is a natural outgrowth of his lame-duck status and the beginning of the 1988 presidential campaign, but the daily pounding of the hearings has accelerated the process.

"There is a growing feeling he is unable to provide vigorous leadership," said Sen. Nancy L. Kassebaum (R-Kan.).

"It has punctured the myth of the Reagan administration and the Reagan term; people now question his policies and his advisers on a whole range of issues," said Rep. Dave McCurdy (D-Okla.). "Rather than be angry, they have basically started to write him off. They don't want to see him tainted or further embarrassed, but at the same time they are ready for a change."We tend to focus on the wrong consequence of the Iran-contra affair," said Rep. Connie Mack III (R-Fla.). "The consequence is that the party and the administration have lost their momentum."

Where "Reagan's agenda" once dominated the political landscape, Mack added, "when you go out now, there is no agreement on what the agenda is or even what it should be."

Whether Reagan can regain much of his lost luster remains open to question. Rep. Barney Frank (D-Mass.) dismissed the possibility. "It's too late and he's too muddled," Frank said.

Republicans such as Rep. Jerry Lewis (R-Calif.) are more hopeful. By the fall, Lewis noted, the hearings will be over and Reagan may produce a "ratifiable" arms control agreement at a summit with Soviet leader Mikhail Gorbachev.

Also uncertain are the consequences for Reagan's party in the 1988 elections. Weber worries that the GOP is caught in "a dangerous game."

"We had a lot at stake in one man's credibility and popularity," he said. "This may have hurt the party more than the president. People like Reagan personally . . . but anything substantive is suffering."

Democrats, having suffered for more than six years under Reagan's dominance, are cautious in assessing the political fallout. "This hasn't cost the Republican Party the presidency in 1988," said Rep. Alan D. Wheat (D-Mo.). "We still have to go out and win it."