Yet there is another yardstick by which candidates measure running mates. Do they bring something to the table? Can they help with a critical voting bloc? Can they bring a crucial state?
Lyndon Johnson is regarded as a brilliant choice by JFK, though his brother Bobby, among others in the Kennedy camp, loathed LBJ. LBJ locked up Texas and helped bring home five other former Confederate states for the Roman Catholic nominee from Boston.
In deciding on a vice president candidate, many considerations have to be running through Mitt Romney's mind.
His choice must be seen as ready to be president or at least able to attain that status in short order, and augment his strength with a strategic constituency or help corral a major state he would otherwise have difficulty winning.
Then there is the iron rule of the Hippocratic Oath: Primum non nocere. First, do no harm. The VP candidate also should be conversant with a panoply of issues, fully prepared to defend the nominee's positions on domestic, foreign and economic policies.
Such considerations suggest that whoever in Romney's camp floated the name of Condi Rice to The Drudge Report last weekend was more concerned with changing the subject from Bain Capital and the Caymans than in signaling where the candidate's head and heart are today.
That Rice is accomplished and competent is not in dispute. But should Romney choose her, within hours we would be re-litigating the Iraq War. It was, recall, Rice who slapped down skeptics of that war by implying their reluctance to invade Iraq might just be risking a nuclear surprise attack on the United States.
"There will always be some uncertainty about how quickly Saddam can acquire nuclear weapons," said Rice. "But we don't want the smoking gun to be a mushroom cloud."
Rice was George W. Bush's leading saleslady for a war that cost America $1 trillion, 4,500 dead and 35,000 wounded, and cost the Republican Party both houses of Congress in 2006 and the presidency in 2008. That war is today regarded by many U.S. foreign policy scholars as among the greatest strategic blunders in American history.
Should Rice be chosen, she will be spending much of the campaign defending her role in that war. And Gov. Romney will find himself defending, or disagreeing with, what George W. Bush did a decade ago.
Rice is by her own admission "mildly pro-choice" on abortion, a position mildly anathema to religious conservatives, the foot soldiers of the party, many of whom have been only lately won over to the governor himself.
Rice's whole career has been devoted to foreign policy. Can she be brought up to speed in weeks to learn and recite the new catechism of the party and defend it from a hostile press or in debate with Joe Biden?
Most Republicans have no idea where Condi stands or what she believes about right to life, gay marriage, affirmative action and the Arizona immigration law. Asked whom she voted for in 2008, Rice reportedly said, "I just want to acknowledge that when the (2008) election took place and after the election took place, it was a special time for Americans."
Did the candidacy of John McCain make it a "special time"?
My friend and former White House colleague Peggy Noonan says that when she mentioned the possibility of Condi Rice as vice president to a gathering of business types, "spontaneous applause burst forth." Condi's nomination, she wrote, would be truly "exciting."
Peggy's got that right. The right is boiling with excitement already.
But would it be wise for Romney, who bears no responsibility for the record of George W. Bush, to choose a running mate who would force him to defend a Wilsonian policy of compulsive interventions across the globe "to end tyranny in our world"?
The choice of Rice would be a Romney endorsement of the Bush foreign policy of which she was co-architect, having spent four years as the national security adviser and four as secretary of state.
Tim Pawlenty could help carry Minnesota. Sen. Rob Portman could help secure Ohio. Sen. Marco Rubio would likely deliver Florida and help in a Hispanic community that is 16 percent of the U.S. population and may in 2012 constitute 9 percent of the vote.
Can Condi Rice deliver California? What does she bring?
When a candidate is facing what seems an insurmountable lead, he will often consider a roll of the dice.
Ronald Reagan's team, 20 points down, considered putting ex-president Gerald Ford on the ticket. Walter Mondale, 20 points down, picked a congresswoman from Queens whom America did not know. John McCain picked Sarah Palin.
But candidates who are running even tend not to take huge risks. Surely there are other ways to shift the subject from Bain Capital.