A candidate's vice presidential pick can either propel him to the election or stop him short. Just look at 2008: When Sen. John McCain selected Sarah Palin to be his running mate, she almost immediately dominated the news cycle and was launched to superstardom from obscurity.

Now that Republican candidate Mitt Romney has secured the nomination and officially started his vice presidential search by tapping aide Beth Meyers to lead the project, the speculation has only intensified.

Will Romney go with a plain vanilla candidate in order to avoid the circus and distraction that occurred in 2008, or will he choose a rising star who will break boundaries? 

The stakes are high for the former Massachusetts governor, who is nearly neck-and-neck with incumbent President Barack Obama in the polls. Here's a look at who might be on Romney's short list, one of Washington's favorite guessing games.

“It’s [Romney’s] decision months from now, not mine. So why spend time thinking about it? … If this bridge ever comes that I should cross it, then I’ll think about it then. It’s not the time to think about it.” – The Wall Street Journal, April 11

Why he’s on the list: The Wisconsin U.S. representative made a name for himself by authoring a budget plan to go up against Obama’s own economic blueprint. As Chairman of the House Budget Committee, he is one of the most influential voices in his party on fiscal issues. Just days before the Wisconsin primary on April 3, Ryan officially endorsed Romney and campaigned by his side for three days.

But … Despite the clout he has among some of his GOP peers, Ryan’s budget plan is controversial and unpopular among independents. He also doesn’t appeal to the far right, which Romney needs in a running mate -- when Ryan rebutted President Obama’s State of the Union address, tea party-affiliated Michele Bachmann crafted her own response, competing with Ryan's.

“I don’t want to be the vice president right now, maybe ever … The bottom line is that I really want to do a good job in the senate. I really believe the senate is a valid place to shape and drive American policy.”

“If in four to five years, if I do a good job as vice president – I’m sorry, as senator – I’ll have the chance to do all sorts of things.” – The National Journal, April 19

Why he’s on the list: The 40-year-old Florida U.S. representative is only a freshman in Congress, but he’s already a star. GOP leaders and Tea Partiers have been buzzing about Rubio and potential vice presidential ambitions since he was elected in a high-profile three-way senate race in 2010. As a Cuban-American from a battleground state, some supporters of the Romney-Rubio ticket (like Jeb Bush) hope he might help win over Hispanic voters who usually lean Democratic.

But … a recent Public Policy Polling analysis found that a Latino running mate may not do much to boost Romney’s Hispanic support. Also, the senator still may be too young and inexperienced for the opportunity.

Nikki Haley

“I’d say ‘Thank you, but no.’ I made a promise to the people of this state. And I think that promise matters, and I intend to keep it.” – ABC’s “Nightline,” April 2

Why she’s on the list: Like Rep. Marco Rubio, South Carolina Gov. Nikki Haley is a tea party favorite and a potential game changer. Not only would she be the first female vice president if Romney gets elected, but she would be the first Indian-American candidate. Sarah Palin endorsed her during her gubernatorial race in 2010 and she heavily campaigned for Romney this year. Haley is particularly known for her strong stance against unions and says she brought 22,000 new jobs to her state since taking office.

But… She’s made it pretty clear she’s not interested. Republican politicos are also worried about drawing parallels to Palin because of their ties.

“I’m so boring I didn’t even know I was boring.” – Washington Post, May 23

Why he’s on the list: U.S. Sen. Portman is the favored VP candidate by a lot of Washington insiders; He’s from Ohio, a critical state in the general election, he’s widely respected by his party and would be a “safe” pick (unlike Sarah Palin, for example, who proved to be a controversial character fighting for the spotlight). Formerly a trade representative and budget director in the Bush administration, Portman has the government experience. And, unlike a lot of other Republicans on this list, he actually wants the job; he enthusiastically endorsed Romney just before the March 6 primary.

But … A recent CNN/ORC poll found he was the least-known among Republicans. By being that “safe” pick, Portman comes across as boring and he lacks the charisma the N.J. Gov. Chris Christie or Florida Sen. Marco Rubio has. He is also not conservative enough for Tea Party voters.

“He might be able to convince me. He’s a convincing guy” – Philadelphia Inquirer, April 30

Why he’s on the list: The popular New Jersey governor kept Republicans on the edge of their seat late last year before finally saying he had no intention of running for president. His supporters love him for his confrontational yet charismatic style of leadership and called for him to run, but said his duty to the people of the Garden State and his family came first. In April, however, Christie said Romney “might be able to convince me” to be his running mate should he ask.

But … The strong personality that his supporters love him for may make the Romney camp shy away from choosing him for a number two spot because he may be overshadowed.

“Somebody who actually wants to run for office would be a good start,” – Rice on who Romney should pick for VP, Sacramento Bee, May 3

Why she’s on this list: In addition to being a game-changer (she’d be the first female African-American candidate) the former Secretary of State under the Bush administration is enjoying high popularity. According to an April CNN/ORC Internation poll, Rice had an 80 percent favorability rating.

But… She’s made it very clear she’s not interested, and other than endorsing Romney in late May, she hasn’t actively campaigned for him like Nikki Haley and Chris Christie have.

“I made a decision that the difference I can make is in the Senate, but I don’t think you ever rule out options and opportunities to serve your country,” USA Today, June 10

Why he’s on the list: John Thune, a junior U.S. Senator from South Dakota, made waves in the Republican Party when he defeated Democrat Tom Daschle for the seat. He endorsed Romney early in his campaign and he might be the usually non-controversial and highly respected running mate that the former Massachusetts is looking for. Thune also has years of Washington experience (he was the House of Representatives before the Senate) and leans farther right than Romney on policy – something that could help the relatively moderate candidate gain enthusiasm among more conservative voters.

But… Democrats have attacked Thune for his unwillingness to compromise, and some voters might be turned off by his extensive experience on the Hill. South Dakota is also an already traditionally Republican state with a few electoral votes, so that wouldn't provide as much of a strategic advantage to Romney's ticket as a candidate from a wing state.