DES MOINES — There are 75 foods on a stick, including apple pie and gluten-free corn dogs. There is a cow sculpted from butter. And there are 18 presidential candidates in a bipartisan stampede to the Iowa State Fair — bringing their stump speeches, of course, but more important, aiming to prove their common touch in the ultimate venue for retail campaigning.

The 11-day fair at this midsummer point in the presidential campaign captures better than anything else the pure spectacle that has overtaken the volatile race in both parties. A Republican debate last week confirmed the disruptive power of Donald J. Trump, and the long-presumed probable Democratic nominee, Hillary Rodham Clinton, is feeling the heat from the enormous crowds drawn to Senator Bernie Sanders of Vermont.

On Saturday at the fair here, Mr. Trump plans to arrive by helicopter, adding to visits planned by Mrs. Clinton and Mr. Sanders. The candidates have to be mindful of what they wear — jeans are in, chinos are out — and make sure they sample the Iowa delicacies, which are often packed with cholesterol. And then there are the hecklers.