Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" and of the new book "Governing America."

(CNN) -- Mitt Romney has taken many people by surprise by announcing that his vice presidential running mate will be Wisconsin Representative Paul Ryan. The decision excites many conservatives who have been calling on Romney to go big. They believe Ryan will inject some juice into a campaign they feel has been lackluster and put the focus on the policy differences between Romney and President Obama.

The primary risk with Ryan, from what we currently know about him, is that his controversial budget plan and tough line on Medicare could energize liberals and alienate elderly voters in key states like Florida. He also lacks foreign policy expertise and has spent most of his career in the city that conservatives hate, Washington. In recent decades, the record of vice presidential running mates who have come right out of the House is not very good.

The risk of making the wrong choice for vice president was highlighted in 2008 by Sen. John McCain's selection of Alaska Gov. Sarah Palin, a decision many believe helped torpedo the Republican nominee. Palin struggled before the media, appearing inexperienced and unprepared for the Oval Office. She also used aggressive conservative rhetoric that undercut McCain's appeal to independents....