Mr. Giuliani has appeared tired since Monday, when he held an unusually subdued town-hall-style meeting in Durham, N.H. But he seemed upbeat when he last faced reporters on Wednesday night outside the fund-raiser in Chesterfield.

His illness comes as his campaign has hit a rough patch. His lead in national polls has been shrinking lately, with several recent polls showing him essentially tied for first place among the Republican candidates after months in which he led more comfortably.

The Giuliani campaign is betting on an unorthodox strategy in which they are not counting on winning the first states that vote, Iowa and New Hampshire, but are instead focusing on winning the Florida primary on Jan. 29 and then trying to win a number of the big states that hold their primaries on Feb. 5.

Mr. Giuliani never had much support in Iowa, which holds its caucuses on Jan. 3. And after he failed to lift his standing in New Hampshire polls with a heavy advertising campaign there, he scaled back his efforts in order to marshal resources for later states. He spent Wednesday campaigning in Missouri, where voters go to the polls some 48 days from now.

The campaign did not immediately alert reporters who were traveling with Mr. Giuliani, but instead they said they sent a bulletin about his admission to hospital to The Associated Press at 1 a.m.

Mr. Giuliani’s hospitalization capped a day when he had unexpectedly had to drive across Missouri after his private jet broke down. “We were going to take an airplane, and when we got on the plane they said, ‘Oh, one engine just went out,’ ” Mr. Giuliani told supporters in Columbia, Mo., Wednesday afternoon after arriving a couple of hours late to his event. “We thought it was better not to take the airplane.”