ROCK RAPIDS, Iowa — In a state that prizes retail campaigning, nothing is more revered than “the full Grassley” — a politician’s visit to all of Iowa’s 99 counties, named for the state’s peripatetic senior senator, Charles E. Grassley.

Rick Santorum became the first presidential candidate this year to complete the marathon with a rally last Tuesday in Lyon County in the northwest corner of the state, a four-hour drive from Des Moines, where grain elevators loom like mountain ranges and FM stations broadcast hog prices and news of Asian trade deals.

Gov. Scott Walker of Wisconsin, who has visited 25 counties, has announced his own plans to complete the circuit, in a state he considers crucial in his path to the nomination.

“People say: ‘Why?’ ” Mr. Santorum told a crowd in Rock Rapids, the Lyon County seat. “The idea is to make yourself available to the people who have the responsibility, as you do, to suggest to the American public who should be the president of the United States.”