WASHINGTON — It happens every four years in the dog days of summer: The presidential candidate — both of them, in some years — picks a running mate.

There’s the buildup, the rabid speculation about who’s being considered, then the all-important rollout, sending the media and the political world into a frenzy.

As Mitt Romney and Rep. Paul Ryan have undoubtedly learned, there are plenty of boxes to check.

Will the vice presidential pick give the ticket geographic diversity? Help the candidate appeal to blue-collar voters? Women? Minorities? How about foreign policy experience, legislative know-how, private-sector skills?

Will any of it make a difference?

“It depends on the top of the ticket, to a great degree,” says Craig Smith, a speechwriter in the Gerald Ford and George H.W. Bush administrations. “The motto is generally ‘Do no damage,’ like Hippocrates and the doctor’s oath.”

With few other opportunities to alter the contours of the race dramatically, White House hopefuls may be tempted to make a game-changing pick.

That’s especially true if, like Romney, they’re down a few points in the polls and finding that, their best efforts be darned, the numbers just aren’t moving in their direction.

But in modern times, VP candidates rarely have helped the candidate lock it up in the home stretch of the campaign. And frequently they’ve been a drag.

A few notable vice presidential selections that didn’t end up deciding the race:

2008: John McCain-Sarah Palin

McCain has been roundly criticized for his selection of Palin, a first-term Alaska governor with an affinity for going off script. Palin delighted the conservative base but struggled to show mastery of many policy issues. The bump in the polls McCain enjoyed after rolling out Palin was short-lived, and the Arizona Republican lost by a convincing margin to Barack Obama and Joe Biden, who peddled his own brand of blue-collar politics. But Palin’s public stumbles were not likely what lost McCain the election. Rather, it was the economy, whose bottom fell out under a Republican president’s watch two months before Election Day.

2004: John Kerry-John Edwards

With his good looks and “Two Americas” tagline, Edwards lent a warm, personal appeal to the ticket. He played an attack dog role against President George W. Bush, but his genteel, Southern manner softened the attacks enough that they didn’t necessarily turn off voters. The pair had a tense relationship, with Edwards harboring ambitions beyond the vice presidency. But in the end, Edwards played less of a factor in the race’s outcome than the Swift Boat attacks that undercut Kerry’s military credentials at a time when the country was embroiled in two wars.

2000: George W. Bush-Dick Cheney

What the Texas governor lacked in legislative experience or national security bona fides, he made up for when he picked Cheney, a former congressman who had served as defense secretary under Bush’s father. The duo defeated Al Gore and Joe Lieberman in Cheney’s home state of Wyoming, but in all likelihood would have won that conservative-leaning state whether or not Cheney was on the ticket.

1996: Bob Dole-Jack Kemp

There may be a lot to be gleaned about Romney’s VP pick from Kemp, a tax-cutting aficionado who counted Paul Ryan among his disciples and employed Ryan as an aide in his congressional office. President Bill Clinton had a hefty lead over Dole in the polls before the senator added Kemp to the ticket. That went a long way toward narrowing that lead. But the economy was still thriving in the mid-1990s, and Clinton’s personal life had yet to implode. Voters may have liked Kemp, but they were content to give Clinton another four years at the helm.

1992: Bill Clinton-Al Gore

It’s hard to dismiss the boost that Clinton got from picking Gore as his running mate. Both were younger than 50, giving the campaign a sense of vivacity and fresh ideas. Clinton’s charisma and empathy were complemented by Gore’s temperate disposition and attention to policy detail. Both Democrats called the South home — a factor that helped them defeat President George H.W. Bush and Vice President Dan Quayle in typically Republican states such as Georgia, Kentucky and West Virginia. Yet their victory in the election may have had less to do with Gore and more to do with Ross Perot, a third-party candidate who split off 19 percent of the vote — much of it from the Republicans.

1984: Walter Mondale- Geraldine Ferraro

Case in point for why modern presidential campaigns typically vet their vice presidential prospects to the high heavens: Ferraro, who was a huge boon to Mondale’s candidacy in the first days after he announced his pick. The first woman to appear on a major party’s presidential ticket, Ferraro came with a compelling narrative about her humble beginnings. But soon came the relentless questions about her husband’s finances, which turned Ferraro into a major liability for the campaign. At the same time, President Ronald Reagan was a popular incumbent, and the economy was recovering. Mondale and Ferraro lost the race by almost 17 million votes — that’s 18 percentage points — and by a 525-13 margin in the Electoral College.