The office of the vice presidency has become a center of real power in recent decades. That could be even more the case in the next administration.

Donald Trump, the Republican presidential nominee, said he was looking for a running mate who could help guide him through the unfamiliar territory of Washington. Democrat Hillary Clinton, who will be formally nominated this week, stated that her main concern was choosing someone who could assume her job, should that prove necessary.

Both turned to governors. Republican Mike Pence is the sitting governor of Indiana, and Democrat Tim Kaine served as Virginia's governor prior to his election to the U.S. Senate four years ago.

That's unusual. With the exception of Sarah Palin in 2008, no sitting governor had served as a vice presidential running mate for nearly half a century.

To understand why governors have emerged as appealing running mates this year -- and why running mates in general are increasingly important -- Governing spoke with Joel Goldstein, one of the nation's leading experts on the vice presidency. Goldstein, a law professor at St. Louis University, has published numerous books and articles on the office, including this year's The White House Vice Presidency: The Path to Significance, Mondale to Biden.

The interview has been edited for length and clarity.

There's some indication that Mike Pence could be an unusually important vice president. Trump has said he's interested in having someone run day-to-day operations while he sets the larger strategic course. There was even a report that when the Trump people approached Ohio Gov. John Kasich, they told him he'd be charged with running both domestic and foreign policy. What's your guess about what Pence's role as vice president would be?

It's impossible to know. On the one hand, Trump obviously needs somebody to help, in the sense that he doesn't have experience dealing with national or international issues. He also doesn't have experience dealing with political institutions or with the people who occupy them. And his leadership style suggests he'll be much more removed.

On the other hand, his personality makes it unpredictable to know exactly who he would work with and who he would delegate to. Pence seems to be the sort of guy who might be effective in dealing with Trump, but it's difficult to know.

The other day, Utah Gov. Gary Herbert said he'd be voting for Pence, and that Trump comes as part of the package. Do you think the running mate matters that much to a lot of people?

The vice president is most likely to make a difference when voters are relatively indifferent to the presidential candidates. Given the nature of Trump and Clinton and the polarization of the electorate, one would expect that wouldn't be the case in this election.

It's possible Pence's selection will be reassuring to some Republicans. It's likely Clinton's selection will send a message about her. Kaine's getting some pushback from the progressives, but that's not totally unusual. Lloyd Bentsen did in 1988, Joe Lieberman did in 2000.

We talked about what sort of role Pence might have in the White House. What about Kaine?

A while back, Jeff Greenfield wrote a piece in Politico arguing that the vice presidency would be worthless because Trump is idiosyncratic and won't listen to anybody, and Bill Clinton would assume the vice presidential role if Hillary wins. But I think Kaine is likely to be important.

There are advantages to having a vice president like Kaine that even somebody like Bill Clinton can't provide. There are things that members of Congress will say to a vice president who they feel comfortable with that they wouldn't say to a former president or a president's spouse. If you're interested in keeping channels open, a vice president can be important.

Kaine's also got unique experience at dealing with mayors, governors and members of Congress, and apparently has been successful at all three roles. If you think about Joe Biden's work heading up the stimulus and working with mayors and governors, Kaine might play a similar role.

We've had a number of governors nominated for or elected president in recent decades but hardly any governors as running mates. Why have they been an unpopular choice, and why are they in fashion this year?

You're right. Sarah Palin was the only gubernatorial running mate selected since Spiro Agnew, the GOP nominee in 1968 and 1972. The other former governor was Nelson Rockefeller, who replaced Agnew as vice president. And Ed Muskie was a former governor in 1968 on the Democratic side.

The last time we had two governors on a ticket was 1948. In part, the reason we don't have governors as running mates is a number of recent presidential candidates have been governors, and they've chosen running mates who were Washington insiders.

You've written about how the vice presidency has grown in importance. Do you have any thoughts on why this hasn't been the case for lieutenant governors? Some are handed portfolios of real authority, but a lot of them still have little to do.

The president chooses the vice president. It's clear that he's the president's choice. But in a lot of states, you have the lieutenant governor elected independently of the governor. 

You have some situations like we have in Missouri where you have a split party. Even where you have two people of the same party, the lieutenant governor is not accountable or doesn't have the same reason for loyalty toward a governor as does the vice president.

And there's a lack of the same foreign policy or national security role. There are a lot of places that need attention. Vice presidents do a fair amount of work on diplomatic matters. Some of it is ceremonial, but much of it is substantive. That isn't part of what the governors have to deal with. 

Getting back to the vice presidency, there's a political scientist named Jonathan Bernstein who says that Joe Biden has been a near-perfect vice president. Do you agree with that?

I don't know if I would say that, but I think he's been incredibly successful. I wrote a piece saying that Biden has been more successful over two terms than any other vice president in history.

He has established a relationship with Obama that has endured and by all accounts strengthened over their term together. Unlike Bush and Gore, Biden hasn't spend most of the second term running for president. Unlike Cheney, he sustained his influence in the second term. He's been incredibly important as an adviser and troubleshooter and spokesperson. 

But defining success for vice presidents is difficult. It's hard to measure because so much of what a vice president does comes in private conversations with the president. It's never known to anyone outside of the two of them -- or to a very small circle.