WASHINGTON — The U.S. Government Accountability Office is opening a review of President Trump’s $28 billion bailout for farmers harmed by his trade war amid allegations that the money was mismanaged and allocated unfairly.

The investigation came at the request of Senator Debbie Stabenow, a Michigan Democrat, who has been vocal in her concern that the aid program was biased, providing more funds to southern states that voted for Mr. Trump and favoring large and foreign agriculture companies over small farms.

The Trump administration, which signed an initial trade deal with China last month, said the farm subsidies would end this year. The program began in 2018 as a $12 billion effort to mitigate losses for farmers who lost sales or faced retaliatory tariffs from China, the European Union, Canada and Mexico as a result of the trade war. The program grew to $28 billion last year as Mr. Trump’s conflict with China festered.

Critics have faulted the program for the formulas it used to determine payments for certain crops and for providing funds to big corporate farms. The program, which used a Depression-era fund, allowed farmers earning less than $900,000 a year to receive money if they produced one of the agricultural products that faced retaliation. The government also purchased certain products, such as apples, oranges and pork.