DES MOINES, Iowa — Dennis DeWitt says he typically doesn’t get involved in politics until it is time to vote, but he took the opportunity here Friday to throw his support behind President Trump in the Iowa State Fair’s popular “corn kernel poll.”

Mr. Trump is showing his strength in the informal survey of fairgoers, which through Thursday night showed that half of the more than 4,700 kernels cast had gone for Republicans and half had gone for Democrats.

Mr. Trump had received support from 97% of Republicans, while former Vice President Joseph R. Biden was the top choice of Democrats, pulling in 33% of the votes cast. The poll will run for more than a week.

“He’s telling the truth that people don’t want to hear, and they don’t like it,” Mr. DeWitt said after dropping his kernel into the Trump mason jar, which was filling up quickly Friday. “He does say things he shouldn’t say, but the economy is good.”

The support for Mr. Trump contrasts sharply with the criticism the 2020 field of presidential contenders has leveled at the Republican at the nearby Soap Box forum.

Sponsored by the Des Moines Register, the event is giving more than 20 presidential contenders the chance to deliver a stump speech and field questions from people at the fair.

The lineup of speakers Friday included former Housing and Urban Development Secretary Julián Castro, entrepreneur Andrew Yang, former Maryland Rep. John Delaney, spiritual guru Marianne Williamson and Rep. Tulsi Gabbard of Hawaii.

The foursome is struggling in the Democratic kernel poll.

South Bend, Indiana, Mayor Pete Buttigieg is running second among Democrats in the kernel poll at 14%, followed by Sen. Elizabeth Warren of Massachusetts, 12%, Sen. Kamala Harris of California, 11%, and Sen. Bernard Sanders of Vermont at 5%.

Former Massachusetts Gov. Bill Weld, who has launched a challenge against Mr. Trump for the Republican nomination, has notched 3%.