U.S. presidential hopeful Sen. Barack Obama, D-IL, stops to speak with school kids from Holy Cross as he departs after a vote on amendments to S.2284, the "Flood Insurance Reform and Modernization Act of 2007," on Capitol Hill in Washington on May 13, 2008. (UPI Photo/Patrick D. McDermott)

Speaking Tuesday in Des Moines, Obama praised fellow candidate Sen. Hillary Clinton, D-N.Y., calling her "one of the most formidable candidates" to seek the presidency, then turned his eye to Sen. John McCain, R-Ariz., the presumptive Republican nominee, The Washington Post (NYSE:WPO) reported.

"This year's Republican primary was a contest to see which candidate could out-Bush the other, and that is the contest John McCain won," said Obama, D-Ill.

Obama traveled to Iowa Tuesday to mark not only his first victory of the primary-and-caucus season but also his securing a majority of pledged delegates thanks to a primary win in Oregon -- a symbolic benchmark his campaign says represents Obama's becoming the party's nominee in November, the Post said.