Mr. Walker, who has fallen in one key Iowa poll from first place in July to 10th place this month, no longer plans to appear next weekend at a prestigious Republican conference on Mackinac Island in Michigan or at the California Republican Party convention. Instead, his advisers said, he plans to campaign in Iowa — where he is holding events this weekend as well — and in South Carolina.

Mr. Walker’s advisers said the last-minute cancellations weare not a sign of panic about the viability of hiss presidential bid but rather a recognition that at this point his time and campaign funds are better spent on Iowa and South Carolina. Mr. Walker regards Iowa, which will hold the nation’s first presidential nominating contest on Feb. 1, as virtually a must-win state that would energize his supporters and donors nationwide. And he has long seen South Carolina, which votes later that month, as another winnable early state that could give him momentum and stature in a large field of Republican candidates.

By skipping the events in California and Michigan, two states with larger and more diverse electorates than Iowa and South Carolina, as well as more delegates at stake to help win the nomination, Mr. Walker risks diminishing himself. Once a national front-runner, he increasingly looks like a regional candidate — hoping his Midwestern roots will win him Iowa — who is pursuing single-state strategies rather than projecting confidence across the country.