President Reagan yesterday stiffened his stance against additional emergency aid to farmers, warning them and their banks that they should not expect the taxpayers to bail them out.

At his news conference Thursday, Reagan offered no increase in his proposal of $650 million in loans and loan guarantees for farmers, but nevertheless voiced sympathy for their plight.

Yesterday, while again expressing sympathy, he used harsher language in rejecting pleas for more assistance, saying that "taxpayers cannot be asked to bail out every farmer hopelessly in debt . . . or be asked to bail out the banks who also bet on higher inflation. We have already extended a tremendous amount of assistance. It's time for others to pitch in and do more."

Reagan's weekly radio address came as the nation's breadbasket was dispatching its forces eastward in what is expected to be an extraordinary display of pressure on the federal government this week.

The 105-member South Dakata state legislature is to be among the Grain Belt legislators, governors and farmers lobbying for federal aid to shore up farmers and rural communities caught in the credit crisis that threatens to bankrupt thousands of farmers this spring.

Friday is when many farmers will be told whether their loans will be renewed for this year.

Yesterday Reagan said that two-thirds of American farmers have no debt problems and that only a fraction of the rest are in "severe financial distress."

His emergency aid program will help the neediest farmers in the current crunch, he said.

He also defended the longer-range restructuring of farm programs he sent to Congress Friday, saying it is time to begin "working our way back to a free-market economy" in which the farmers will have "less dependence on politicians to supply their incomes." The government's "inflexible" farm programs have "weakened incentives for self-reliance" and "sent false signals to the market," he said.

The administration's five-year plan to phase out farm supports would mean "government will stop purchasing commodities, stop trying to manipulate supply and demand, refrain from quick-fixes and extravagant new farm legislation and move aggressively to expand markets for American farm products," Reagan said in the broadcast from his Camp David, Md., retreat. He added, "The time is now for a fresh start for American agriculture."

This week's massive lobbying effort began a few weeks ago as a gleam in the eye of state Sen. Tom Vickers, a Nebraska farmer and rancher.

After his call for legislative delegations to come to Washington, 15 states agreed to send elected representatives to lobby.

South Dakotans reacted so eagerly to Vickers' call that they decided to send the entire legislature and help finance their trip with a public-subscription "give-a-buck" campaign.

Vickers said Friday that the delegation's intention is to impress on official Washington the need for quick credit assistance -- more than the White House has offered -- and to make proposals for longer-term farm policy that might put agriculture on a more stable footing.

The state delegates have arranged a Monday meeting with Vice President Bush but say they still are hoping to meet with Reagan.

"We already know what [Agriculture Secretary] John Block thinks. He has decided he wants to be the last secretary of agriculture. We know what [budget director] David Stockman thinks about agriculture, and even his mother doesn't agree. We hope to get eyeball to eyeball with the president . . . . We're afraid that watching 'The Beverly Hillbillies' and 'Hee Haw' tells him all he wants to know about rural life," Vickers said.

The state legislators' appearance here, however, is just one portion of a massive campaign to pressure Congress and the administration to deal more forcefully with credit needs, farm policy and rural development.

Agricultural issues will figure heavily on the agenda of the winter meeting of the National Governors' Association, which began yesterday. Another farm policy conference, sponsored by the Monsanto Co., will bring together several hundred state officials, members of Congress and farm leaders to discuss farm legislation.

Meanwhile, more than 200 farm and rural community organizations Tuesday are to hold a news conference, headed by former agriculture secretary Bob Bergland, to criticize the administration's fiscal 1986 budget proposals that would cut rural programs severely.

Bergland, executive vice president of the National Rural Electric Cooperative Association, also has scheduled another news conference Tuesday to outline farm policy proposals of the Center for National Policy, a Democratic-oriented public-interest group.

Congress, for its part, has scheduled at least four hearings this week to hear testimony from the visiting state legislators and farmers.

And a credit package adopted last week by the House Agriculture Committee, adding more money to the Reagan administration's debt-restructuring program, is to move to the floor for a final vote, with easy passage expected.

The filibustering farm-state senators who delayed the confirmation vote on the nomination of Edwin Meese III as attorney general indicated that they may attempt to force a Senate vote on the House credit bill. They propose to attach it as an amendment to an appropriations measure that provides $1 billion in famine relief for Africa and $1 billion more for U.S. farm loan guarantees.

Waiting in the wings, with a weeklong lobbying campaign scheduled to begin March 4, is the American Agriculture Movement, which sponsored the 1979 tractorcade to protest declining farm prices.