Last Thursday morning, on a farm just outside Des Moines, independent candidate John B. Anderson had an opportunity to inject a little electricity into his faltering presidential campaign. Anderson missed that opportunity, and missed it in a way that suggests why he is faltering.

The morning was perfect -- as clear as newly washed windows, cool and bright. One of Anderson's best advance men had arranged for the use of Fred and Richard Higginbottom's big equipment barn in Maxwell and had filled it with local farmers, more than 200 of them. Anderson's staff had provided the candidate with a good speech to deliver on farm policy for the occasion.

But Anderson squandered his chance. He wandered pointlessly from his prepared text. He sounded preachy to the crow. He failed to convey a crisp, clear message that would have matched the morning weather. After 20 minutes the crowd got restless. A lot of the men reached for their cigaretts and lit up. Anderson had lost his audience -- if he ever had it.

In fact, he probably never did have the audience. Interviews with farmers in the crowd indicated that most of them came from curiosity, not because they supported the candidate. Most appeared to favor Ronald Reagan. Even the Higginbottom brothers, who had volunteered the use of their farm for the event, said they hadn't decided yet whom they vote for.

Things haven't turned out as Anderson hoped when he declared his candidacy as an independent in April. Instead of tearing themselves to shreds the Democrats have achieved a workable level of party unity, depriving Anderson of potential supporters and removing the chance that a truly prominent politician would join the Anderson ticket.

Once the Illinois congressman dreamed of convincing Gov. Hugh Carey of New Ork of Sen. Daniel Patrick Moynihan (D-N.Y.) to run with him for vice president; now he must settle for former governor Patrick Lucey of Wisconsin.

When Anderson suddenly blossomed as a political phenomenon last winter, he projected a new sort of image. He was to be an honest man for the '80s, a straight-talking candidate who would carry bad tidings to the voters if he had to. This was an opening for an entire campaign strategy, but that strategy never materialized.Indeed, very little new has materialized since the New Hampshire and Massachusetts primary campaigns. Anderson often sounds the same today as he did then.

The plan had called for raising enough money in the spring and summer to put Anderson ads on television during the Republican and Democratic convention. But the campaign couldn't raise that much money. It has raised about $15 million. President Carter and Ronald Reagan will each have nearly $30 million in federal funds for their campaigns.

Anderson is an un distinguished stump campaigner, as that lost opportunity in Iowa last Thursday attests. Anderson evidently is aware of this shortcoming. He hinted as much during a visit to the Iowa State Fair.

One stop on his tour of the fairgrounds was at the pen housing the winner of the state fair's Biggest Boar contest, a 904-lb. named Big John.

With the television cameras taking it all in, Anderson tried to banter with Big John's owner. Was it true the hog was named for him, Anderson asked?Oh, yes indeed, said the farmer.

"I see," the candidate replied. "Well, you know, there's some people who think if you spell 'boar' a little differently, then I might not have to take second place to this prizewinner." Anderson does bore some audiences. Unlike most candidates on the stump, he had no single pep talk or collection of jokes or set of themes that he repeats at every stop.

Instead, each rally, fund-raiser or media event evokes from Anderson a new presentation. On one level this is refreshing, but on another it conveys the essence of Anderson's difficulty this summer: He doesn't seem to know what he wants to say.

Those farmers outside Des Moines who listened to Anderson got nothing simple and straightforward from his speech that they could grab on to, take home and tell their neighbors about. And this is the way it is for most Anderson audiences.

Yet Anderson on the stump these days regularly encounters crowds yearning to be excited by him -- supporters already committed to his cause. Anderson appeals to a category of his countrymen that might be called Cleancut America. This week he has been meeting representatives of Cleancut America from Maine to california, and at every stop they look the same: mostly earnest and idealistic.

Sometimes, these supporters' enthusiasm draws some excitement out of the candidate. It happened in Chicago on Wednesday at the opening of a new Anderson headquarters in the loop. The room was steaming and overcrowded, but emotion was high, and Andersn responded with a good speech and some of his best one-liners of the week. For example:

"The [Carter] slogan used to be, 'Why not the best.' Now it's, "Well, he isn't the worse."' The crowd love it, and laughed. So did Anderson. But he didn't use the line again all week.

Here in Minneapolis 2,500 mostly young Anderson supporters turned out for a highly successful rally in the city auditorium. But despite their determination to be enthused, Anderson let the crowd down. He responded well at first, and 10 minutes into his talk was beginning to make the most of the emotion in the hall. Then he got bogged down in dissertations on domestic policy, calling for "improved transportation for the rural and semi-rural areas," and declaring solemnly:

"We can put people back to work. And that's how I'm going to solve the economic problems of this country -- with jobs!"

Such vague assurances are not uncommon from Anderson. He says often that in the campaign this year he intends to speak forthrightly on the great issues that face the Republic, but in fact he rarely does so. Instead, he often talks like an old-fashioned politician.

So, when he spoke in Chicago to the Veterans of Foreign Wars, Anderson declared that the country's military establishment is dangerously weak and proposed improvements that he said would cost "billions;" before his 2,500 Cleancut American supporters in Minneapolis, he lambasted excessive defense spending and "militarism."

In an interview on WHO Radio in Des Moines -- the station where a sportscaster then known as "Dutch" Reagan got his start -- the interviewer asked Anerson if he were president and the Soviet Union moved troops into Poland, would he send American forces to Poland's defense?

"We're not in a position to do it," Anderson replied. "We're not strong enough. Let's face it . . . we're in bad shape."

Did he really mean to imply that the United States should defend Poland in such a situation if it was strong enough? The press corps pressed the question the next chance it got, and Anderson neatly backed away. He didn't think that this country would have any business defending Poland against the Soviets, he said.

"I merely meant to point out in my answer to that question," he explained, "that it seems to me it's almost academic in view of the defense establishment of this country at the present time . . . ."

"The key to reinvigorating Anderson's campaign," said a man this week who has been working hard for him since last winter, "is for Anderson and his advisers to recall what launched him in the first place: The impression hundreds of thousands of people got that they could trust him, that he wouldn't coddle or fool them, but that he'd tell the truth. Unless he recaptures that, he's gone."

But if he fails to recapture it, will he really be gone? One of the interesting aspects of the Anderson phenomenon is the support he already has in Cleancut America. Wherever he goes now, he sees people who honestly, enthusiastically want him to be president of the United States.

Last April, the day after he declared his independent candidacy, Anderson was asked what he would do in October if he realized then that he had no chanmce of winning the election, but might just throw it to Ronald Reagan.

"I would always hope that I would make the kind of judgment that would be good for my country, and that I would put aside my ambitions and my career and do what as right," he replied.

If the country really "had to choose between the lesser of two evils," Anderson added, he would do what was necessary to make sure it really got the lesser evil.

But with Cleancut America cheering him on, walking away from the fray won't be easy for Anderson.

"I'll still only be 58 years old" in November, he noted wistfully on an airplane this week. "It won't be the end of my life."

Or the end of his ambition either.