Republican presidential candidate Mitt Romney is drawing a disproportionately high amount of campaign funds from people who have given him the legal maximum, weakening his ability to raise money from such proven donors as the GOP nomination fight drags on.

Far more than his three rivals, Mr. Romney has tapped these large donors to build his campaign fund. To date, about two-thirds of his money has come from donors who have given him $2,500, the legal limit for a primary campaign,...