All presidential nominees pay lip service to the ideal of choosing the individual who is best qualified to assume the presidency if fate were to so dictate. But always present also is a pure calculation of seeking to strengthen the ticket politically or shore up a perceived weakness.
Such matters as ideological and geographical balance, gender, ethnicity and potential to capture a large chunk of electoral votes all get thrown into the pot. And sometimes when a presidential nominee feels a Hail Mary pass is necessary, desperate selections are made. Remember Geraldine Ferraro by Walter Mondale in 1984, Dan Quayle by the senior George Bush in 1988, and Sarah Palin by John McCain in 2008.
In all three cases, there was no clear evidence that the choice made any real difference in the outcome. Ms. Ferraro, as the first woman nominated for national office, made no dent in the Ronald Reagan landslide re-election. Mr. Bush was elected over Michael Dukakis despite Mr. Quayle's serial gaffes. Ms. Palin's early flash was dampened by revealed deficiencies, and she couldn't prevent Barack Obama's historic election.
But over than last 20 years, the winning presidential nominees -- Bill Clinton, George W. Bush and Mr. Obama -- have chosen individuals whose long public service recommended them as reasonably qualified to assume the presidency if called on: Al Gore, Dick Cheney and Joe Biden. One may argue that one or all failed to live up to that potential, but the solid track record was there.
In each case, the running mate proved to be more than a show horse, brought not only into the political but also the policy-making deliberations of the administration to which he was elected. The partnerships developed on the campaign trail were carried over into governance of the country -- for good or evil, depending on one's own perceptions.
So the experience of the last two decades weighs heavily on Mr. Romney, if he is nominated, to choose his running mate on the basis of true qualification to assume the presidency if so required, and to play a productive role in helping the president govern the country in the meantime.
In eight years as vice president, Mr. Gore proved to be an effective right-hand man to Mr. Clinton over a range of domestic matters. Mr. Cheney in eight years under Mr. Bush became a powerful voice in the shaping and implementation of foreign policy. And Mr. Biden for nearly four years has been an influential figure for Mr. Obama in both realms, as a former chairman of both the Senate Judiciary and Foreign Relations Committees.
Whether Mr. Romney will follow suit in his selection of a running mate obviously depends on his own perceptions of what role he wants that individual to play. The fact that he himself has had little experience with foreign policy or governing on the national level would suggest he will seek to shore up those deficiencies, but with someone personally compatible, to assure a smooth working relationship.
The current flavor of the month, freshman Sen. Marco Rubio of Florida, would seem a poor fit. So would any of the Republican governors and former governors being mentioned, or Rep. Paul Ryan of Wisconsin, unless Mr. Romney would be willing to bet his chances on Mr. Ryan's budget plan, an obvious campaign target for the Democrats.
In terms of being a partner in governance, Sen. Rob Portman of Ohio, as former U.S. trade representative and budget director under the junior President Bush, would seem to have the credentials. It also helps that comes from the state that no Republican has ever won the presidency without carrying.
All this is no guarantee that a Mr. Romney running mate would not become another Ferraro, Quayle or Palin. But Mitt seems nothing if not cautious and serious. That alone would be a hopeful augur for selection of someone who really would be qualified for the presidency, and useful as vice president as well.
Jules Witcover is a syndicated columnist and former long-time writer for The Baltimore Sun. latest book is "Joe Biden: A Life of Trial and Redemption" (William Morrow). His email is firstname.lastname@example.org.