Speculation has been ramping up over who Republican presidential candidate Mitt Romney will choose as his vice-presidential candidate, as a top campaign advisor suggested that his pick could come at the end of the week.

On Monday, The New York Times wrote that Romney had in fact made his pick. But senior Romney adviser Eric Fehrnstrom said that he hadn't finalized his decision.

Asked specifically whether Romney could announce his vice presidential pick this week, Fehrnstrom said: "Technically it could, but the governor hasn't made a decision. It will only happen after he makes a decision."

Names have been bandied around for months — from outspoken New Jersey governor Chris Christie to the young and popular Florida Senator Marco Rubio.

But many believe Romney will play it safe — opting for Ohio Senator Rob Portman or former Minnesota governor Tim Pawlenty — both solid conservatives with a broad appeal but neither considered particularly politically dynamic.

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Many believe Republican presidential candidate Mitt Romney will play it safe when it comes to making a vice-presidential pick. (Evan Vucci/Associated Press)

Yet the political prognosticating was stirred up again last week, sparked by the screaming headline on the Drudge Report that former secretary of state Condoleezza Rice was now potentially in the running.

Observers immediately jumped in, questioning whether Rice, who has in the past emphatically denied any ambitions to jump back into politics, was truly being vetted or whether her name was being floated out there as a trial balloon.

No matter — the pundits were soon out in full force weighing in on the pros and cons of a Romney/Rice ticket.  

There had been some speculation that Romney may choose early, eschewing the traditional time of picking around the convention. Reuters reported last June that the team had discussed whether to announce the pick a few weeks earlier than expected to generate buzz for his campaign during August and help raise campaign funds.

Romney may have other reasons to announce early. It would provide a needed distraction from the barrage of attacks from the Obama campaign over his time at Bain Capital and repeated calls to release his tax records. 

Romney will also be heading to London for the start of the Summer Olympics before travelling to Israel and then, possibly, a rumoured foreign policy trip through Europe, which means if the announcement doesn't come this week or next, it will  probably be put off until convention time around the end of August.

The importance of the vice-presidential pick continues to be debated but most agree that while the choice may not add to a campaign, it certainly shouldn't be a political liability. This is why Romney is expected to make the safe choice, to avoid the so-called 'Palin factor'

The prevalent wisdom is that in 2008, former Alaskan governor Sarah Palin hampered Senator John McCain's campaign. Although she seemed to re-energize the conservative base when she was first announced as McCain's running mate, questions about her qualifications soon emerged following a number of media interviews, including an infamous chat with CBS News' Katie Couric.

Whether Palin actually had a significant impact on the electoral fortunes of McCain is hard to say. In 1988, former president George Bush Sr. got criticized for his choice of Dan Quayle, who was also accused of lacking experience. It seemed to make little difference as Democratic presidential candidate Michael Dukakis was crushed by Bush.

A study by Bernard Grofman and Reuben Kline, political scientists at the University of California, Irvine, evaluated the impact of vice presidential selection on voter choice. It found that the net impact of vice presidential selection is at most one percentage point in total votes. And in 2008, the impact was about one-half of a percentage - slightly lower than the historical average.

As for running mates helping to carry their home states, Karl Rove, political strategist and former senior advisor to George W. Bush, also pointed to research by political scientists Christopher Devine of Ohio State and Kyle Kopko of Elizabeth College argue the home-state advantage is often modest.

While 2000 and 2004 races came down to one state, 2000 (Florida) or 2004 (Ohio), in neither of those instances did either party field someone from those states, Rove observed in an op-ed piece for the Wall Street Journal.

"A running mate's principal political impact is on behalf of the presidential candidate's themes or issues. The vice-presidential candidate helps reinforce what the presidential candidate is emphasizing. But if the top banana on the ballot isn't getting it done, the running mate won't be able to on his or her own," Rove wrote.

"Choosing a running mate reveals much about the presidential candidate himself. Though still only a candidate, this is his first presidential decision."

With files from The Associated Press