Leading Republicans have begun taking some experimental swings at President Carter to find out how vulnerable the Bert Lance affair has left him.

In what aides billed as the first comprehensive critique of the administration by a top elected Republican office-holder. House Minority Leader John J. Rhodes (Ariz.) charged yesterday that the White House has been marked by "uncertainty and ineptitude, compounded by moral blindness and a lack of understanding of the processes of government."

"I very much dislike saying this, but I believe that it needs to be said. If there is one strand of behavior discernible through the fabric of the Carter administration's policy, it is the strand of duplicity."

"A president who received his party's nomination with words that the American people are tired of seeing 'big shots' evade the consequences of their actions, now appears to millions of Americans to be singing a very different song," Rhodes said in remarks prepared for delivery to the Commonwealth Club in San Francisco.

Republican National Chairman Bill Brock used a similar tack at a breakfast with reporters Thursday, saying he sees the Lance affair as "the beginning of a public perception that the President is really not in control of the situation - that he has not yet commanded the strength, the continuity or the logic" promised in the campaign.

While the remarks from the two leaders seemed to be testing a theme for possible use in the 1978 congressional elections, the party leadership says it has not hit upon a surefire strategy to turn the Lance affair to political advantage.

At the Republican National Committee, party officials insisted firmly that they have not even sought ways to capitalize on Lance's fall from power, saying they will step back and let the impact of the resignation unfold by itself.

Pocketbook issues - unemployment, taxes and what they call the Carter administration's failure to generate a growth economy - will be the key issues in the 1978 congressional campaigns, they said.

But, whether spontaneously or by design, the party leaders have sketched within their pronouncements on Lance's misfortunes the skeleton of a strategy that could erode the public's perception of Carter's moral stature.

By itself, disclosure of Lance's freewheeling financial dealings would provide little grist for next year's congressional campaigns, Brock said. Nearly a year's time between Lance's departure from Washington and the 1978 campaigns will have dimmed the public's consciousness of the controversy, he surmised.

But, coupled with such controversists as the one surrounding Carter's endorsement of the cargo preference bill, which Republicans allege is a $600 million expression of gratitude for heavy campaign contributions by maritime unions, the Lance affair could have added significance, Brock said.

"If Lance is symptomatic of a pattern perceived by the people (that) changes their estimate of Carter's moral stance, the combination could be a political advantage," Brock said.

Also, Brock said, Carter set a different moral standard for his close friend, Lance, than he promised in the campaign he would set for all public officials.

The result, at the minimum, will be that "people will be more cautious in their judgment of him [Carter]. People will place a different weigh on the words 'morality' and 'propriety,'" Brock said.

A national Committee official later sought to minimize the committee's concern with Lance as an issue, saying that it has not even been a strategy topic at recent meetings.

"It's too early to tell. Future developments will tell whether this is an issue. Right now, we're not pursuing it, "the official said. He said the committee was actively pursuing the cargo-preference issue.

Meanwhile, in the first step toward giving the party the voice and force of a shadow government the committee has created three panels of experts and Republican dignitaries to formu campaign strategy.

With a better financial position - late positions on national issues, which will form the framework of $9.1 million raised already this year for congressional races, compared with only $1.4 million in 1975 - the GOP National Committee and the state chairmen will meet next week in New Orleans to discuss issues and coordinate efforts to rebuild the party from the city hall and statehouse level up.

How to get the most mileage out of theCarter administration's missteps is also expected to be a topic of considerable discussion during the conference.

In what may be more a reflection of confidence than a measure of reality a recent survey by the Christian Science Monitor showed that three-fourths of GOP state chairmen and committeemen who answered a questionnaire said they now see Carter as vulnerable to a Republican challenge in 1980.

Many of the same leaders, while vigorously critical of Carter, previously had indicated in Monitor surveys that they felt the President was so popular that he could not be defeated if he ran again.

Rep. Guy Vander Jagt (Mich.), chairman of the Republican Congressional Campaign Committee, says he believes Carter's handling of the Lance investigation will demonstrate that the President "is no different than anyone else despite his lofty campaign statements."

The Lance affair has caused a "daily hemorrphage on the credibility of Jimmy Carter," and the effect will be an erosion of Democratic strength in Congress next year, Vander Jagt in Congress next year, Vander Jagt said in a recent meeting with reporters.

Vander Jagt said the White House "managed to take something bad and make it worse," and he suggested the public attention will help offset the Republicans' Watergate stigma because voters next year will "rememberthat we were taken to the woodshed and punished."