Democrats said the choice of the Wisconsin architect of a failed House budget bill is a sign the Republicans are catering to the far right. Republicans, on the other hand, hailed Ryan as a courageous and fiscally responsible politician.
Romney, the former Massachusetts governor and presumptive Republican presidential nominee, made the announcement in Virginia as he began a tour through several swing states.
"Romney's choice demonstrates that catering to the Tea Party and the far-right is more important to him than standing up for the middle class," Senate Majority Leader Harry Reid, D-Nev., said in a written statement.
U.S. Rep. Debbie Wasserman Schultz, D-Fla., chairwoman of the Democratic National Committee, joined the chorus, the Miami Herald said.
"The American middle-class and our seniors cannot afford to go back to the failed policies of the past, and it is unconscionable to ask them to foot the bill for the Romney-Ryan scheme," she said. "A Romney-Ryan ticket is sure to take us back and repeat the same catastrophic mistakes that got us into the mess we found ourselves in the first place."
The American Bridge 21st Century, which The New York Times called a Democratic political organization, said Ryan's strict budget proposals would be a campaign issue regardless of his role in the race. "So why not set the task of defending the plan to the architect himself, Paul Ryan," the group said in a written statement.
"As a member of Congress, Ryan rubber-stamped the reckless Bush economic policies that exploded our deficit and crashed our economy," Messina said. "Now the Romney-Ryan ticket would take us back by repeating the same, catastrophic mistakes."
Vice President Joe Biden praised Ryan up to a point, ABC News said: "He's a bright, handsome guy from the state of Wisconsin. He a fine guy. But I think his ideas are not nearly as fine as he is a man."
Republicans cheered the selection. Sen. Rob Portman, R-Ohio, who had been considered a top candidate for the vice presidential nomination, said Ryan was one of his best friends in Congress and had his support.
"I look forward to working closely with a Romney-Ryan administration to restore fiscal sanity and enact pro-growth policies to create jobs," Portman said.
U.S. Sen. Marco Rubio of Florida, the only person Romney acknowledged considering for vice president, called Ryan "a courageous reformer who understands our nation's challenges," the Herald said.
"The Romney-Ryan ticket is going to win in November because it offers the American people visionary leadership to recapture the free enterprise spirit that has empowered countless Americans to build businesses from scratch and live the American dream," he said. "I'm excited about the visionary change a Romney-Ryan team will bring to Washington, and I look forward to campaigning with them this fall."