The American Agriculture Movement's second annual attempt to jar official Washington out of its urban complacency begins today when farmers come chugging into the city on their tractors again.

Emboldened by success in lobbying for farm legislation last year, AAM delegates say they want still higher price supports for their produce.

They add that the power structure still hasn't taken cognizance of the costs borne by 2.3 million farm families in producing more than enough food for 220 million Americans.

This is almost the same message as last year, but conditions both in Washington and in the farm country have changed in ways that could complicate the lobbying task of the farmers.

"The gut feeling is that it's a tougher road this year," said a congressional aide who follows farm issues.

Since last year, the Carter administration has declared inflation to be economic enemy No. 1. A year ago at this time, retail food prices were predicted to increase by only 4 to 6 percent. Now the prediction is for an increase of 7 to 10 percent in 1979.

Officials say that urban legislators who were inclined to give farmers a break last year may be less ready to do so now.

The elections are now over and rural legislators may feel under less immediate pressure to help farm constituents.

New farm legislation is still in place from 1978. It includes improved government credit provisions for farmers and a guaranteed average wheat price that comes to $3.40 a bushel with subsidies.

According to nationwide statistics, U.S. farmers had a fairly good year in 1978.

Net income was $28 billion, up $8 billion from 1977. And assets of farmers increased from $700 billion to $800 billion.

Livestock and dairy producers fared particularly well. Omaha prices for live steers have increased from $39 per 100 pounds three years ago to more than $60 per 100 pounds now.

The situation is "a heck of a lot better than a year ago for farmers," says Lamar Spangler of the American Bankers Association agriculture division.

These factors have raised doubts about the ability of the tractorcades to get what AAM wants this year: an increase of price supports for all commodities to 90 percent of parity. Parity is a measure of farm income based on farm costs and living standards just before World War I.

Wheat and corn prices would have to rise by several dollars a bushel to reach the 90 percent of parity AAM is seeking.

The American Farm Bureau Federation, which says AAM does not represent the U.S. farm community, has kept at arm's length.

"AAM did accomplish something last year by focusing the attention of city people on agricultural problems," said the Farm Bureau's John Datt. "But there isn't the sympathy that existed a year ago. If they start charging around town, they'll do more harm than good."

Secretary of Agriculture Bob Bergland has promised a warm reception for the visiting farmers. His department is making a reception room and telephone service available.

But Bergland told the Senate Agriculture Committee last month that he doesn't intend to seek major changes in farm legislation.

At the same hearing, however, Bergland gave a clue as to why farmers are again demonstrating here this year. The statistics, he noted, are averages and don't give the full picture. Some farmers are "hurting," he conceded.

Grain producers, who make up a sizable segment of AAM's activists, have not done nearly as well as live-stock and other producers in the last few months.

AAM's John Eberle, a North Dakota producer of durum wheat, said the 500 acres he farms produces sales of $70,000 a year. "I'm living on escalating land values," he says.

Nationwide, farm costs have been rising rapidly, from $52 billion in 1972 to $95 billion last year. And farm profits still have not come near to the record $33 billion in 1973.

Bergland has stressed that inflation and rising energy costs hit farmers harder than almost any other segment of the economy. At the same time, he noted that the largest increases in food prices come from rising processing and merchandising costs rather than farm prices.

Although there are 2.3 million farms earning $1,000 or more in the United States, most of the people owning them have stopped counting on farming as their sole livelihood.

Four out of five of these farms sell under $40,000 worth of produce a year. To supplement income from farming, these farms depend on $34 billion earned elsewhere.

Nonfarm income of farmers now exceeds the profits from agriculture by a considerable amount.

According to congressional observers, AAM's core of members is people who still are trying to earn all or most of their living from farming, and finding the going difficult.