Ryan is "a right-wing ideologue" who wants to "lavish" the wealthy with tax cuts while cutting spending on programs that help the middle class, students and senior citizens, David Axelrod, a campaign strategist for President Barack Obama, said on ABC's "This Week."
"He wants trillions of dollars of more budget-busting tax cuts skewed to the wealthy," Axelrod said during an appearance on NBC's "Meet the Press." "This is a prescription for economic catastrophe."
Republicans countered that Romney showed leadership and courage by picking Wisconsin's Ryan, who as chairman of the House Budget Committee has pushed efforts to scale back the size of government through spending reductions and tax cuts. He is most identified with his proposal to replace Medicare, the health-insurance program for the elderly, with a plan offering seniors a fixed amount of money for private insurance.
Ryan, 42, was an "extraordinarily exciting choice, because you now have a national leader who is capable of talking in detail with the American people about some very complicated topics, and that's a very unusual moment in American history," former House Speaker and ex-Republican presidential candidate Newt Gingrich said on CBS's "Face the Nation."
Sen. John Thune, a South Dakota Republican, said on CNN's "State of the Union" that Ryan's selection shows Romney is "very serious about doing big things for this country." Ryan represents "everything conservatives had hoped for," Thune said.
Ryan is "good matchup" because of his "intimate knowledge" of the budget, McCain said on "Fox News Sunday."
"This is a substance-driven campaign versus a fear-and-smear campaign," Ed Gillespie, a senior adviser to Romney, said on CNN.
Democrats said they welcomed the debate, and portrayed a Ryan-authored budget plan as benefiting the wealthy with tax cuts while cutting back programs that help the middle class.
"The debate has sharpened," Chris Van Hollen, a Maryland Democrat who serves on the Budget Committee, said on CNN.