WASHINGTON – Five wealthy people, led by Dallas industrialist Harold Simmons and Las Vegas casino mogul Sheldon Adelson, have donated nearly $1 of every $4 flowing to the super PACs raising unlimited money in this year's presidential race, a USA TODAY analysis shows.

Those donations have helped new Republican-leaning outside groups swamp Democratic-friendly super PACs in fundraising — money that is used largely for attack ads. The large sums also have rejuvenated the underfunded campaigns of principal challengers to former Massachusetts governor Mitt Romney in the race for the GOP nomination.