By SARA MURRAY, COLLEEN MCCAIN NELSON and PATRICK O'CONNOR

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GOP presidential candidate Mitt Romney, left, jokes with Rep. Paul Ryan (R., Wisc.) after announcing him as the 'next PRESIDENT of the United States' during an event introducing Mr. Ryan as his running mate in Norfolk, Va. on Saturday.

NORFOLK, Va.—Mitt Romney picked Rep. Paul Ryan as his running mate, a decision that could spark enthusiasm for the Republican ticket among conservatives and all but ensures the election will turn to deep philosophical divisions between the two parties over spending, taxes and entitlements.
In Mr. Ryan, 42 years old, the Romney campaign gets a conservative who has spent recent years at the center of national debates about the size and scope of the federal government. With his proposals to revamp entitlement programs for future retirees and the poor, he has become a hero to conservatives and a target for liberals.