text: The choice of a running mate is one of the most crucial decisions a candidate will make during a presidential run.
  He is looking for a number two who can balance his weaknesses, appeal to new voter constituencies, persuade independent voters to give him a second look, and campaign effectively on his behalf.
  All the while, Mr Romney will need a partner who will not outshine him on the stump or otherwise detract attention from him.
  The perfect VP pick will also need to convince voters he or she is ready to assume the presidency if anything happens to Mr Romney - to be just a heartbeat away from the Oval Office, as the American political adage goes.
  As Mr Romney nears a decision, here are some of the top possibilities:
  Paul Ryan, 42, has made a name for himself as the top Republican on budget issues, calling for a dramatic reduction in entitlement programmes and other government spending and in taxes.
  The Wisconsin congressman, chairman of the House Budget committee, is seen as a persuasive surrogate with the capacity to take the Romney campaign on the offensive on issues that are likely to dominate in this election year - such as jobs, the economy and the deficit.
  He is also a staunch conservative from a state that both parties consider to be up for grabs in the coming election.
  Since becoming chairman of the House Budget committee in 2007, Mr Ryan has worked on shaping an alternative vision for the federal budget.
  But Mr Ryan's budget proposals are seen as so severe and conservative that they could pose a problem for Mr Romney among independent voters less keen to slash taxes and cherished social programmes.
  Mr Ryan studied at Ohio's Miami University and was elected to the House of Representatives in 1998 at the age of 28. He is currently serving his seventh term.
  Rob Portman, 56, is a veteran of Washington, having served in the House of Representatives for 12 years and in White House roles under both Presidents Bush.
  In selecting him, Mr Romney would hope to pick up votes from the crucial battleground state of Ohio, while also having a competent veteran of Washington on the ticket.
  But Mr Portman's strengths are also his weaknesses. Often portrayed as dull, he brings little excitement and pizzazz to the ticket. His years in Washington would detract from Mr Romney's effort to run as a political outsider. And his role in the second Bush White House could turn off both independent voters and hard-core fiscal conservatives angered by Mr Bush's expansion of government spending and his widening of the federal budget deficit.
  Mr Portman was born in Cincinnati and educated at Dartmouth College and the University of Michigan law school. Later, he worked in legal roles in George Bush Sr's White House. He served in the US House of Representatives from 1993 to 2005, when George Bush Jr appointed him US Trade Representative.
  In 2006, Mr Bush appointed him director of the Office of Management and Budget, a role in which he helped formulate White House budget and tax policy. He left that job in 2007.
  In 2010 he was elected to the Senate, succeeding Republican George Voinovich.
  Tim Pawlenty, 51, is seen as a "Sam's Club Republican" for his supposed appeal to working and lower-middle class voters (Sam's Club is a chain of giant discount stores run by Wal-Mart).
  Mr Pawlenty made his own brief run for the Republican nomination, but quit the race in 2011 before a vote was counted when he failed to gain traction with voters or the media.
  He could help balance Mr Romney's wealthy, patrician image and certainly brings enough political experience to the table. Currently without a full-time job, he has been an effective surrogate for Mr Romney on the campaign trail, and the men get on well personally.
  But he is seen as lacking charisma, and he does not bring to the ticket a geographic constituency - Minnesota is a solidly Democratic state, having last voted for a Republican presidential candidate in 1972, and before that, in 1956.
  Mr Pawlenty has also broken from current Republican orthodoxy on a number of issues, including on energy and environmental policy (although he later renounced his backing for "cap and trade" plans), and with a call for universal health insurance for children.
  Mr Pawlenty grew up in a family of modest means in South St Paul, the son of a trucking company worker. He studied law at the University of Minnesota and was elected to the state House of Representatives from a suburban district in 1992.
  He rose in the state House, winning the role of Republican majority leader in 1999. In 2002, he was elected governor. He was mentioned as a possible vice-presidential running mate to John McCain in 2008.
  Marco Rubio, 41, is everything Mr Portman and Mr Pawlenty are not: He is young, handsome, energetic, and a gifted and engaging public speaker and television presence. He is a darling of the conservative populist Tea Party movement, giving him a grassroots appeal Mr Romney's other options lack.
  The son of Cuban immigrants, Mr Rubio could conceivably dent Mr Obama's overwhelming advantage with Hispanic voters. And he could help carry the crucial state of Florida for the Republican ticket.
  But Mr Rubio also brings drawbacks. At 41, he is younger than Mr Obama was in 2008, and he has less experience on the national political stage than the president did when he launched his run for the White House.
  When Mr Rubio was young, his family briefly attended a Mormon church, which would risk alienating some evangelical Christian voters already suspicious of Mr Romney's Mormon faith.
  And it is unclear the extent to which Mr Rubio could win Hispanic votes from the Democrats. The majority of Hispanic Americans have a Mexican background and are unlikely to identify with a Cuban American merely because they speak the same language.
  Mr Rubio, a lawyer by training, was elected to the Florida House of Representatives in 2000 and rose through the leadership to become House speaker in 2008. In 2010 he was elected to the US Senate.
  Bobby Jindal, 41, was the first Indian American elected governor. In choosing him, Mr Romney would hope to broaden his appeal beyond non-Hispanic white voters - or at least soften the party's image among independent voters.
  Mr Jindal is a die-hard social conservative and is leading the Republican governors' charge against President Barack Obama's healthcare reform law. Those attributes could win Mr Romney support on the Republican right-wing.
  Mr Jindal is also fiercely intelligent, attending Oxford University as a Rhodes scholar.
  Mr Jindal's first foray into national politics fell flat - he was chosen to give the Republican rebuttal to Mr Obama's 2009 State of the Union address - and he was dismissed as not quite ready for prime-time.
  His national profile subsequently rose in the wake of the 2010 Deepwater Horizon oil spill, when he positioned himself as a bold leader in dealing with the disaster and a critic of the Obama administration's response.
  Before entering politics, he worked briefly for consulting firm McKinsey and Company in Washington DC.
  While in his mid-20s he was appointed to lead the Louisiana Department of Health and Hospitals, and soon was running the national Bipartisan Commission on the Future of Medicare. Under President George W Bush, he was assistant secretary of the Department of Health and Human Services.
  Mr Jindal was elected to the House of Representatives in 2004, and became governor in 2008.
  John Thune, 51, has been seen as a giant-killer by fellow Republicans since he ousted the Senate's Democratic majority leader to claim his seat in the chamber in 2004.
  The two-term Senator has become a familiar face on US talk shows and is a prolific fundraiser within the party, using extra cash from his own coffers to bolster the campaigns of other Republican Senate candidates.
  Mr Thune's staunchly conservative voting record, especially on social issues, could be an asset for Mr Romney, who has struggled to convince his party's base that he is conservative enough for them.
  In 2010 Mr Thune toyed with the idea of running for president himself but ultimately decided not to, saying in February 2011 that he preferred to serve in the "trenches" of the Senate.
  Mr Thune is the son of a Norwegian immigrant and hails from Murdo, a small South Dakota town west of the Missouri River. He studied at Biola University in California and the University of South Dakota's business school.
  He served in the House of Representatives from 1997 to 2003.
target: US Republican presidential candidate Mitt Romney is expected to name his vice-presidential running mate soon - possibly within days.