Nearly 14,000 midwestern farmers rallied here today to assail Reagan administration farm policies and call for more help in fighting the debt crisis afflicting the Farm Belt with bankruptcy and farm foreclosures.

It was one of the largest rallies in a wave of winter protests and demonstrations as financially hard-pressed farmers seek credit to finance spring planting.

Farmers from Kansas, Iowa, Illinois, Colorado, Ohio, Oklahoma, Nebraska, Missouri, Minnesota, North Dakota, South Dakota and elsewhere journeyed across the nation's slowly thawing crop lands to the Hilton Coliseum at Iowa State University here. They clapped, cheered and whistled in support of calls for a rural political and economic movement to save their way of life.

The final speaker, Jon Wefald, a stem-winding prairie-style Minnesota orator, earned their warmest embrace when he zeroed in on Office of Management and Budget Director David A. Stockman and blamed both national political parties for agriculture's sorry state.

"Forget David Stockman!" Wefald cried to a thunderous roar from the crowd. "The Republicans and the Democrats let us down!

"We will win! We will win!" Wefald concluded, as the crowd shouted back, shaking the coliseum's rafters.

High on their wish list is a return to 100 percent parity on farm price supports, and they derided the administration's advocacy of a "free market" for agricultural commodities. With their pained expressions and emotional pledges of solidarity across fence lines and state lines, they underscored their view of farmers as a minority shoved aside by a country that no longer cares who grows its food or where it is grown.

Naioma Benson, a Colorado farm activist, touched on the Farm Belt's broader anxieties.

"We are feeling insurmountable fear, nearing desperation," she said. "A generation of farm and ranch children are facing loss of home and spirit. All the forces are out of control, altering our lives, our income, our mental health and our entire futures.

"We wanted something better for our children and grandchildren. We need you in urban America to join us in a plea to President Reagan to change the current philosophies he and his administration are following."

Under banners bearing slogans such as "Farms Not Arms" and "Out of Meat? Eat Stockman," the farm folk made clear their fury at Reagan, Stockman and Agriculture Secretary John R. Block, who said yesterday that he would urge the veto of any farm bill exceeding administration requests.

The sense of rage boiled over early. Carlos Polit, an assembly-line worker at an International Harvester tractor factory in Rock Island, Ill., said the union work force there has declined from 3,800 in 1980 to 250. He warned that rural anger and frustration is high.

Reagan "ain't seen nuthin' yet!" Polit cried, echoing the president's inaugural speech last month and setting off a stomping, whistling, standing ovation in the packed tiers of the octagonal hall.

More than a dozen speakers sustained the pitch through four hours of pleas, harangues and analyses. They included three farmers, a banker, a rural businessman, rural activists and several representatives from national consumer and farm organizations. Most of the Iowa legislature and Gov. Terry E. Branstad (R) attended.

Branstad is to meet with the president Thursday in Washington to urge formation of a bipartisan commission to map a long-term national agriculture policy.

Robert Hoffman, who farms 400 acres in Diagonal, said he had come because he is looking for a way to keep his operation going after 30 years and has grave doubts about the future. He said he has joined the Teamsters, an unusual step, "just so I can have another voice."

Building a new rural coalition is a main aim of the 10 rally sponsors, which include the American Agriculture Movement, the National Farmers Union, Farm Crisis Committee, Iowa Farm Unity Coalition, National Catholic Rural Life Committee, the National Rural Electric Cooperative Association and other rural activist groups.

The keynote speaker, Roman Catholic Bishop Maurice Dingman of Des Moines, said, "We are turning the countryside into another version of industrial America -- where there is no room for family or community or the larger interests" of the nation.

Speakers called for quick refinancing for farmers seeking loans for seed and fertilizer. Many blamed farm problems on low commodity prices.

"The main problem facing agriculture is not a lack of credit; it is a lack of net farm income," said Dean Kleckner, head of the Iowa Farm Bureau Federation, which is perceived by many smaller farmers as staunch friend of corporate agriculture.

"They didn't say nuthin'," Hoffman said after the rally, "that ain't the truth."