A few days ago a White House aide remarked, only half in jest, that the first months of the Carter administration could be viewed as a highly successful and systematic effort to alienate key political constituencies. He mentioned specifically labor, blacks, farmers and the American Jewish community.

Today, President Carter leaves Washington on a cross-country trip that will bring him face to face with some of those constituencies. He is not a President in deep trouble, a [WORD ILLEGIBLE] Lyndon Johnson or Richard Nixon expecting to confront mobs of protesters. But for the first time in his presidency, Carter may encounter some signs of discontent with the direction of his administration.

In Des Moines, $50-a-couple tickets to a Jefferson-Jackson Day dinner Carter will address tonight are selling briskly. But as measured by the polls, the President's popularity in Iowa has plummeted along with farm prices. Iowa farmers, according to state party chairman Ed Campbell, still like Carter but have a sense of "anxiety" over the administration's farm policies.

In Colorado, farmers and ranchers are still angry over the cancellation of three water projects, and local newspapers are reporting the possibility of protests by Denver area ilispanic Americans over the President's proposals to deal with illegal aliens.

And when he reaches California Saturday night to address a $1,000-a-plate fund-raising dinner. Carter may feel that he is visiting a foreign country with which the United States enjoys somewhat unfriendly relations. Among other things, the President will be greeted by picket's representing more than 40 activist organizations who will be protesting administration policies on issues ranging from abortion to the neutron bomb.

About the only stop Carter plans in the next two days where tranquility would seem assured is the headquarters of the Strategic Air Command outside of Omaha. But even there, there may still be lingering - if unspoken - resentment over his decision to cancel production of the B-1 bomber.

"People are reading too much into this trip," a senior White House official said yesterday, nothing that its original purpose was for the President to attend the Democratic National Committee dinner in Los Angeles and that the other stops were built around that.

Nonetheless, because of the timing of the journey and the number of issues likely to be raised during it, the President's trip to five states that he lost last November may provide some clues to the state of the Carter presidency.

Energy will be the predominant topic. In the midst of seeking to extract an acceptable energy bill from Congress. Carter will talk about energy at every opportunity, particularly at tonight's speech in Des Moines, according to his aides. But there are other issues, and other political problems, that he will have to deal with along the way. Among them are:

Urban policy: Last month, Carter said he expected shortly to endorse the so-called Humphrey-Hawkins full employment bill, which has the strong backing of the Congressional Black Caucus. However, negotiations between the White House and congressional supporters of the bill have dragged out. Although the two sides are described as "very close," the President, when he attends a roundtable discussion on urban policy in Detroit today, may not yet be in a position to endorse the legislation, which is of prime importance to urban blacks.

Farm policy: Farmers throughout the Midwest are unhappy with the administration's farm bill, and with the decline in farm prices. In Iowa, Carter's approval rating, as measured by the Iowa Poll, dropped from 80 per cent in April to 63 per cent in August. Among farmers, the approval decline was even sharper, from 85 per cent to 56 per cent.

Water policy: One of Carter's first actions in office - the attempt to kill 18 water projects - struck hard in the West, where water is crucial to the economy. Resentment over that has lingered and may surface when the President attends a roundtable discussion on water policy in Denver.

And finally, there is California, where controversy seems to flourish and where the President may encounter the most intense and varied discontent.

The organizers of the California protest, principally the Americans for Democratic Action call their rally "Century Plaza II." Century Plaza I was a tumultuous antiwar rally against President Johnson in front of the same Los Angeles hotel a decade ago.

A number of independent demonstrations also are planned, including a tractor parade by Imperial County farmers protesting the reclamation law and a protest by Jewish organizations unhappy with Carter's Mideast policy.

Underlying the protests is a serious and continuing conflict between the Carter administration and the administration of California Gov. Edmund G. (Jerry) Brown Jr.

This conflict is both political and substantive. Brown beat Carter in the primaries in which the two men faced each other, and he is viewed by some White House aides as a potential presidential rival in 1980. The prevailing view among Brown intimates is that Carter is at best indifferent about the west, which was swept by President Ford in 1976, and that he is hostile to California.

The energy issue serves as the backdrop for the most important policy difference between the Brown and Carter administrations.

The White House, particulary Energy Secretary James R. Schlesinger, has been pressing the California Air Resources Board to approve a massive oil terminal is Long Beach harbor that has been proposed by Standard Oil of Ohio (Sohio) as an alternative to transporting Alaska oil through the Panama Canal. Sohio proposes to pipe the oil to Texas through a pipeline that now carries natural gas to California.

The Air Resources Board is headed by Tom Quinn, who directed Brown's campaign for governor in 1974.

Quinn says he is "an American first and a Californian second" but insists that the terminal will not be approved unless Sohio agrees to expensive "tradeoffs" to counteract air pollution that the facility will cause and unless the Carter administration agrees to guarantee the state an adequate natural gas supply and authority to control the tankers bringing Alaskan oil into Long Beach harbor.

"To allow these oil companies to make a few more dollars at the expense of the health and well-being of half the people in California is outrageous," Quinn said in a recent interview. "Creative people in Washington just wouldn't want to locate an oil terminal in Long Beach."

Carter also suffers from other problems in California, not the least of which is that he is still relatively unknown. Democratic Party officials tend to see him as an outsider who is insensitive to state party problems, a view that was confirmed when the scheduling of the Carter dinner here forced postponement of a fundraiser that was supposed to benefit Democratic legislative candidates.

Beyond the rhetoric lies the threat of a Democratic division in 1980, either behind Brown or a protest candidate.

There already are indications that a protest candidate within the Democratic Party might be funded by Jewish leaders who find Carter's supposed "evenhanded" policy on the Middle East disturbing.

A few blocks-away from the Carter dinner, a $1,000-a-plate "counter-dinner" has been organized by Phil Blazer, publisher of Israel Today and a member of the Democratic National Finance Committee.

Participants will be asked to pledge $1,000 to a congressional candidate who has demonstrate strong support for Israel. The featured speaker will be retired Maj. Gen. George J. Keegan Jr., the former Air Force intelligence chief who stepped down last January and who urges hard-line opposition to the Soviet Union.

Reportedly, the protests by Jewish groups have slowed Carter dinner ticket sales. Democrats here are now saying the dinner will be a success because it will raise $500,000 but a month ago they were claiming that it would bring in $1 million.

One influential Democrat estimated that 600 paying diners will attend and that another 150 or 200 tickets will be distributed free to raise attendance.