Vice-presidential candidates can be divided into two categories: political choices selected for what they can deliver on Election Day and governing picks who can do some heavy lifting in the White House.
By choosing Tim Kaine, Hillary Clinton will get both.
Clinton has been the favourite to win in Virginia since the Republicans chose Donald Trump, whose backing in heavily populated suburbs such as Northern Virginia should lag behind the previous three Republican presidential candidates.
But with Kaine on the ticket, Virginia can probably retire its swing-state jersey for this year. A good-size Democratic victory in this once-reliable GOP state should be expected, likely larger than President Barack Obama's 3.9 point margin in 2012. Recent studies have suggested that a solid running-mate can add two to threepercentage points in his or her home state.
Kaine will also add more than home-state votes.
Experience matters greatly to success on the campaign trail and in office, and Kaine has experience at every level. He has spent more than two decades learning local, state and federal government through service as a city council member, mayor, lieutenant governor, governor and US senator - all the while never losing an election. Even Republican politicians have acknowledged that Kaine mastered each job, and many have praised Kaine's savvy and collegiality.
Kaine's critics on the liberal side have called him boring, prompting Kaine to acknowledge this "flaw". Actually, that's true only if you consider a keen mind and intellectual rigor to be dreary. Kaine can slice and dice policy like the Harvard Law School grad he is. Plus, if there's ever been a year when we've needed less excitement, it's this one.
Naturally, there will be downsides to Kaine's nomination. Some of Bernie Sanders's voters are surely disappointed, having hoped for an outspoken liberal such as Senator Elizabeth Warren. Kaine will not shake up the race or cause people to think about Clinton in new ways. And in a year when many Americans appear dead set against long-term officeholders, Kaine's record will cut against the grain.
In this hyper-partisan era, Kaine is a bit of a throwback to the days when pols at least tried to work together. When he was elected lieutenant governor, he went to see every one of the 40 senators scattered around the state, without fanfare, to get to know them and find common ground. When he ran for governor in 2005, he hoped to continue a bipartisan approach, but Republicans in the legislature had no desire to cooperate with another Democrat, having helped the previous Democratic Governor, Mark Warner, achieve high popularity through passage of his transportation programme. Kaine was frustrated in accomplishing much of his agenda.
Yet Kaine has continued to reach out. As soon as he arrived in the Senate, he tried to make common cause with as many Republicans as he could. To an outsider, the Senate seems mainly inclined towards gridlock, but when I recently asked Kaine what proportion of his chamber was open to working together, he unhesitatingly (and optimistically) answered "75 per cent". That's where Kaine may be disappointed again as a vice-presidential nominee. Does considerable common ground exist anymore?
Kaine isn't much of an attack dog, which was widely noted in his joint appearance with Clinton last week. Kaine isn't timid, and he learned a lot about targeting opponents as the chair of the Democratic National Committee for part of Obama's term. Still, he'll never be as savage as many Democratic partisans would like. It isn't in him.
What is in him is a philosophy in clear contrast with that of his Republican counterpart, Indiana Governor Mike Pence. Both men are substantive and low-key, but Pence is a staunch conservative, while Kaine is more liberal than his reputation in progressive circles suggests. Those who call Kaine a moderate forget that his early career took place in a Virginia that was far more conservative and traditionalist than the demographically diverse state of today.
Looking at Pence's and Kaine's lengthy records, the contrast is sharp.
To the extent that Kaine focuses on Pence rather than Trump, he will be certain to make an issue of the religious freedom law Pence signed as governor, which was widely viewed as an attack on the lesbian, gay, bisexual and transgender community. Kaine has a lengthy record of backing gay rights.
The debate about climate change, where Kaine and Pence take opposite sides, is another sharp difference. The Pence-Kaine disagreements even extend to smoking. One of Kaine's proudest achievements as governor was a smoking ban in bars and restaurants; by comparison, Pence has questioned whether smoking actually causes cancer.
If Clinton wins, Kaine could be an enormously valuable vice-president. Moreover, one can easily imagine Kaine as president if it ever should prove necessary. In recent history, this hasn't always been the case with running-mates.
Like all vice-presidential candidates, and occupants of the nation's second-highest office, Kaine will have to defend some things he would never have done personally and believes are wrongheaded. The real question is whether Kaine, viewed by friends and foes as a moral, ethical person, can find his own voice within a Clinton circle that has sometimes made poor decisions and taken self-defeating shortcuts. Could Kaine do more than be a good team player by guiding Clinton to better choices?
We are about to find out - initially on the stump and maybe in the next administration. First, Kaine will have to grit his teeth and help win an unpleasant scorched-earth campaign.
- Larry Sabato is director of the University of Virginia's Centre for Politics and author of The Kennedy Half-Century.