To conservatives, it's an electrifying choice that puts the slumping economy and growing deficit front and center, but plenty of Democrats are giddy, calling Ryan an extreme choice to form a team that will put a greater burden on the middle class and seniors.
President Obama met with some of his top campaign people today in Chicago after the Romney announcement.
The president arrived in the Kenwood neighborhood after eating dinner at a North Side restaurant and meeting with staff at his campaign headquarters in the Loop.
As the president arrived on Air Force One, the Ryan pick had Republicans here excited.
"He's young, he's youthful, he has a lot of energy and I think he's going to spark a lot of buzz," said Lori Yokoyama of the Chicago Republican Party.
In a statement, Illinois' GOP chair said, "Congressman Ryan is one of the best and brightest minds in the country and a leading voice for the economic reforms this country so badly needs."
The 42-year-old congressman is known as a true fiscal conservative, proposing trillions of dollars in budget cuts and wholesale changes to Medicare and social security.
"Medicare is about to go broke," Yokoyama said. "It needs to be revitalized. It needs to be revised."
"Conservatives love it because of his budget hawk position," ABC7 political analyst Laura Washington said. "Democrats love it because they see him as being the most vulnerable to criticism."
In a statement, Vice President Joe Biden said he called Ryan to congratulate him and looked forward to engaging him on the clear choice voters face this November.
"Ryan is known for his pursuit of a way too austere and scary budget, particularly in regards to student loans and opportunities for students to go to college," said Lt. Gov. Sheila Simon.
The Romney campaign is already moving to downplay Ryan's position on entitlements cuts according to talking points sent to Romney surrogates obtained by ABC News.
Sunday on Twitter, Obama strategist David Axelrod wrote: "Wow. Mittsters break land speed record trying to distance from radical Ryan budget. Problem: it looks just like Mitt's!"
"If you want to attack workers and workers' rights, then Romney and Ryan would be your team," Rev. Jesse Jackson said.
The president has a total of five fundraising events on Sunday before he leaves for Iowa on Monday for the start of a three-day bus tour.