Running mates are vital – but not in the way you might think

[Update: This story was updated on May 5 at 11:20 a.m.]

With Donald Trump now in a commanding position to take the Republican presidential nomination, buzz is building about one of the most consequential decisions his campaign will have to make: the choice of who should join his ticket as a vice presidential aspirant.

In public interviews this week, Mr. Trump sounds ready to cast a broad net in his search​. ​​He's opened​ the door to considering​, among others,​ both Sen. Ted Cruz of Texas and John Kasich of Ohio, the two rivals who ran against him longest in the primary elections this year.

But for both Trump and for likely Democratic nominee Hillary Clinton, some lessons of history are worth pondering. First, even a carefully chosen running mate isn't likely to make or break their campaigns, which will rise or fall mostly on their merits. Second, ​the selection may ​matter most​ vitally because of ​the ways a vice president can help after an election victory.

“It hasn’t been shown that the vice president helps [win] in a lot of cases,” says John Fortier, of the Bipartisan Policy Center think tank. “Primarily, people vote for the top of the ticket.”

But the moment of choosing does give voters an early glimpse of a nominee’s decision-making process, and it can help around the margins. That could be particularly meaningful for Trump, who has never held elective office, has alienated large swaths of voters, and who lags behind Mrs. Clinton by 13 points in a recent CNN/ORC poll.

For Clinton, who also is struggling with high negative ratings but is in the stronger position, the priority must be to “do no harm” when choosing a running mate, says George Edwards, a political science professor and expert on the presidency at Texas A&M.

Trump, on the other hand, “needs so much,” Mr. Edwards comments. He needs someone who not only knows the issues but also someone “who may make people more comfortable and inclined to vote for him.”

Even so, Edwards doubts there is much that, say, a minority candidate could do to help Trump, “because he’s spent months creating this perception of himself and reiterating these strong views” on issues like immigration.

Trump now has the nomination all but clinched. On Tuesday, Republican National Committee Chairman Reince Priebus declared Trump the party’s presumptive nominee.

Clinton, despite rival Bernie Sanders' five-point win in Indiana, still holds a very strong lead in the delegate count, and her campaign says she is on track to win the nomination. This allows the Trump and Clinton campaigns to focus more on uniting their parties and competing against each other – and to devote more time to the running-mate selection process.

According to news reports, Clinton is looking for someone who can help her win and help her govern – and who will understand the influential role her former-president husband is likely to play.

Trump, in a recent interview with The New York Times, says he wants someone with “a strong political background” who can help him on Capitol Hill and also be a good president should the need arise.

When asked if he was surprised that a wide variety of potential running mates did not want to join him on the ticket, Trump answered that he didn’t care. Potential running mates such as South Carolina Gov. Nikki Haley, who might help counter his negatives with women and minorities, and former campaign-trail rival Jeb Bush, who might help him in the swing state of Florida, are not willing to run with the bombastic billionaire.

“Whether people support or endorse me or not, it makes zero influence on the voters. Historically, people don't vote based on who is vice president. I want someone who can help me govern,” he told the Times.

He’s right, according to experts. Trying to deliver a campaign by choosing a candidate with the right geographic or demographic background usually does not work.

Female running mates Geraldine Ferraro (1984) and Sarah Palin (2008) did not push candidates Walter Mondale or John McCain over the top. Political scientists go all the way back to John F. Kennedy’s election in 1960 to find a case where a running mate’s home-state status played a significant role, with Lyndon Johnson helping Kennedy win Texas.

“Joe Biden was not selected to bring in Delaware any more than Dick Cheney was selected to bring in Wyoming, but because the then-candidates felt that they would add to the administration and wanted them as an intimate adviser. That is the new norm,” says Edwards.

The No. 2 job has gained in importance over the last four decades, beginning with the election of Jimmy Carter in 1976.

The southerner broke with tradition and brought Vice President Mondale into the West Wing, giving him an office just steps from the commander-in-chief. Mondale appreciated the proximity, describing the vice presidential suite in the neighboring executive office building as tantamount to an office in Baltimore.

President Carter saw the advantages of a close partnership, explains Joel Goldstein, an expert on the vice presidency at Saint Louis University School of Law.

Carter felt it was simply wrong to leave the second-in-command in the dark, as President Franklin Roosevelt had done with Harry Truman regarding the atomic bomb, Professor Goldstein says. Roosevelt died while in office, before the conclusion of World War II, leaving Truman to come up to speed quickly on the bomb and decide on its use.

Mondale also gave Carter “someone who had really strong interpersonal skills, who had strengths that Carter needed. He knew Congress. He knew Washington. He was well liked,” says Goldstein. “Mondale and his people really came up with the mission of the office – the general adviser and the trouble shooter.”

In the decades since, the importance of the vice president has grown, as evidenced in staff, portfolio, and media presence.

Under President Clinton, Al Gore took on the environment and deregulation as special areas of interest. Dick Cheney played an outsize role in both national security and with the news media during the presidency of George W. Bush. And Vice President Joe Biden is sometimes referred to as “the McConnell whisperer,” for his White House negotiations with his former Senate colleague, Mitch McConnell, the Republican Senate majority leader from Kentucky.

“When you have trust between the vice president and president, it really enables the vice president to do things that nobody else can do,” says Goldstein, such as break bad news or suggest a different approach.

So what might this suggest is most needed in a Trump or Clinton running mate?

Experts interviewed for this story say Clinton would be most helped if she could find someone who can counter her “untrustworthy” negatives, navigate around Bill Clinton, and who has experience governing. Picking a fresh face who lacks experience would be a mistake, says Mr. Fortier.

Trump could benefit from someone with governing and policy experience, much the way outsider governors have relied on insider running mates.

“He could help himself with someone who is knowledgeable about public policy. If that person happened to come from [battleground states] Ohio or Florida, all the better,” says Edwards, at Texas A&M. He suggested Trump could use someone in the mold of Mr. Cheney. “There’s a guy with all the background in the world.”

Both candidates should be keeping an eye out for someone to help unify their party and whom they respect enough to accept criticism from.

In the end, though, there is no silver bullet to picking the right running mate.

“Candidates have multiple needs,” explains Goldstein, “and there’s no one running-mate option that can respond to every one of their needs.”