Mitt Romney's choice of Paul Ryan as his running mate seems to have produced a rare bipartisan moment of glee for Republicans and Democrats as both seem overjoyed by the decision.

The reasons, of course, are different. The Republicans have become re-energized by the pick of the fiscal hawk Wisconsin congressman while the Democrats believe Ryan, the author of two controversial budgets calling on changes to entitlement programs, will be a giant political liability.

But whatever one says about the merits of Romney's choice, the addition of Ryan to the ticket may have a major impact on the race, transforming what seemed to be an ideologically listless campaign into a substantive issue-related contest.

"We've gone from a Seinfeld election about nothing to a meaty discussion on the budget," Larry Sabato, professor of politics and director of the University of Virginia Center for Politics, told CBC News in a phone interview.

Sabato said while the election is still generally about the economy, it will now zero in on things like the debt, the deficit, and taxes and spending in the future. He said it's difficult to say how a debate about Medicare reforms will play out politically.

"On the surface, Democrats should gain because discussions about Medicare almost inevitably peel off the over 65 [year-olds] and they're the most likely to vote. But I don't think it's necessarily a good position to be in to be defending the status quo given the problems we face."

Foreign policy expert and university professor Walter Russell Mead, writing on The American Interest blog, welcomed Ryan's entrance into the field, saying his selection has "made this a better election, clarifying the issues and giving the country something more consequential than attack ads and gaffes to think about."

'Ideological leader' of Republicans

Ryan, the chairman of the House budget committee, has been referred to as the "ideological leader" of the Republican party, and has repeatedly called for tax cuts and entitlement reform, blaming both parties for creating a fiscal mess. Long before his name was announced as Romney's running mate, Democrats had slammed his budget plans to slash the deficit and partially privatize Medicare.

"He is so identified with a singular economic approach that polarizes people. I don't recall a time when somebody was chosen to be the running mate who was so identified with a particular approach that was so polarizing," St.. Louis University law professor Joel Goldstein, considered one of the top experts on the vice-presidency, told CBC News.

Goldstein said other running mates have been chosen in the past to placate the so-called Republican base. But he said that he doesn't ever remember a time where somebody has been picked "who so defined the ideas the ticket was then going to run on."

"Dan Quayle was a conservative Republican but there wasn't a Quayle plan that the Republicans in the Senate had voted on that was the singular issue," he said.

Goldstein, who thought Romney would go with a "safer" choice like former Minnesota governor Tim Pawlenty or Ohio Senator Rob Portman, said Ryan is a "surprising conventional" pick.

"It's conventional in the ideological and experience ticket balancing," Goldstein said. "It's surprising given that it identifies Romney with the Ryan plan, makes him its owner and I think creates an historic situation where the fundamental economic discussion is going to be over a plan that is primarily identified with the running mate."

Goldstein had thought Romney may bypass Ryan as a running mate so as not to be linked to Ryan's controversial proposals.

'Needed to roll the dice'

"What it suggests is that Romney thought he needed to roll the dice a bit, that he couldn’t simply play it safe. Things weren’t working, the electoral map wasn't favourable. If  he was going to get to 270 [electoral votes] he had to really energize his base and had to shake something else up," Goldstein said.

However, Goldstein said what isn’t necessarily a shock is that a Republican presidential candidate made a bold pick.

"One thing that's interesting about the Republican side is that the choice is always surprising. [Dan] Quayle in '88, [Jack]Kemp in '96, [Dick] Cheney in 2000, [Sarah] Palin in 2008, Ryan in 2012. It almost would be surprising if the Republican pick isn't surprising."

The impact Ryan will have on Romney's candidacy remains to be seen. Many believe the vice-presidential pick has little impact on the political fortunes of the presidential candidate.

A study by political scientists at the University of California, Irvine, that evaluated the impact of vice-presidential selection on voter choice, backs that view. It found that the net impact of vice-presidential selection is at most one percentage point in total votes. And in 2008, when John McCain chose Palin as his running mate, the impact was only about one-half of a percentage — slightly lower than the historical average.

Goldstein said while it's difficult to determine what impact a vice-presidential choice has, he rejects the idea that the pick makes little difference.

"One of the things that's so difficult about this, is you never get to play out the counterfactual. It's why it's so problematic to say what impact does the vice-presidential candidate have. Because really it's relative to your other options.

"If your options are Ryan or Herman Cain, I'd be willing to bet a lot of money if he picked Herman Cain his numbers would be worse than picking Ryan."

Goldstein said that some critics point to unpopular vice-presidential choices in the past that seemed to have little impact on the race.

"People liked Lloyd Bentsen had misgivings about Dan Quayle, but Bush/Quayle still won ... The problem with that is the fact that Bush and Quayle won, doesn't mean that Quayle didn't hurt Bush. All it means is that Quayle didn't hurt Bush enough to keep him from winning," Goldstein said.

So far, polls on Ryan's impact on the campaign seem inconclusive. One USA Today/Gallup poll showed that 42 per cent of those surveyed thought Ryan was a "fair" or "poor" choice, compared to 39 per cent who thought he was an "excellent" or "pretty good" choice.

But a Washington Post/ABC News poll showed that before Romney's announcement, Ryan had a 33 per cent unfavourable rating compared to 23 per cent favourable. However, after Ryan was chosen, his favourable rose to 38 per cent while his unfavourable decreased to 32 per cent.

"He's not known yet in the general population. And that's really the key question. Who is going to win the battle to define Paul Ryan?" Sabato said.