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Watch The Polls For Clinton’s VP Bounce. If She Gets One.

We’ll be reporting from Philadelphia all week and live-blogging each night. Check out all our dispatches from the Democratic convention here.

PHILADELPHIA — This is the bounciest time of the campaign season.

Donald Trump has clearly gotten a polling bounce out of his convention. How long that will last may be complicated by a number of factors, including any bounce Hillary Clinton might get out of her convention here. But before we even begin to measure Clinton’s convention bounce, we need to watch out for her VP bounce.

Why would picking Sen. Tim Kaine of Virginia as her running mate give Clinton a bump in the polls? Most vice-presidential announcements, like conventions themselves, have provided a polling bounce to the candidate in question. Look at the press reaction after the Kaine announcement: Clinton grabbed the headlines on Friday night, and Kaine’s first speech as Clinton’s running mate, on Saturday, was heavily covered.

The media reaction to Kaine is typical. Take the fairly successful rollout of Sarah Palin as John McCain’s running mate in 2008. (It’s easy to forget, because of all that came afterwards, that the Palin pick looked pretty successful in the beginning.) Her announcement as McCain’s No. 2, immediately after the 2008 Democratic convention, pumped up Republican voters and short-circuited a news cycle that had been focused on then-Sen. Barack Obama’s nomination party. McCain rode this momentum into his convention and took a lead in the polls.

But it’s difficult to measure vice-presidential bounces because running mates recently have typically been unveiled right before each candidate’s convention (see both selections this year). Still, I’ve looked at the polling immediately before and after every running-mate selection since 1984 to get the best idea possible.

1984 D Walter Mondale Geraldine Ferraro +2
1988 D Michael Dukakis Lloyd Bentsen +2
1988 R George H.W. Bush Dan Quayle
1992 D Bill Clinton Al Gore +12
1996 R Bob Dole Jack Kemp +11
2000 D Al Gore Joe Lieberman +9
2000 R George W. Bush Dick Cheney +3
2004 D John Kerry John Edwards +7
2008 D Barack Obama Joe Biden +1
2008 R John McCain Sarah Palin +4
2012 R Mitt Romney Paul Ryan +3
Median +3.5
Mean +5.4
Don’t forget about the VP bounce

Quayle was announced at the convention. Lieberman and Palin were announced in between the two major-party conventions which makes determining their exact impact more difficult. Not including them in this analysis does not have a significant effect
on its results, however.

Some candidates that we remember now as mediocre selections, such as John Edwards in 2004, seemed to provide a large and immediate campaign boost. The Edwards selection occurred three weeks before the convention, which itself didn’t provide much of a lift to John Kerry. Other VP candidates, such as Lloyd Bentsen in 1988, who provided one of the most memorable moments in debate history, barely gave their running mates a jump in the polls at all.

What kind of boost will Kaine provide? It’s hard to know. My guess — and it’s just a guess — is that there won’t be a substantial Kaine bounce. He was an expected pick that occurred at an expected time in a race in which Clinton doesn’t seem to be underperforming the “fundamentals” (structural factors that affect the race like the economy). Moreover, Kaine isn’t exactly loved by progressives, some of whom have been holding out their support from Clinton. Then again, even Dick Cheney gave George W. Bush a 3-percentage-point bounce.

What Kaine could do is help to halt Trump’s convention bounce. If a Kaine bounce does occur, expect to see the polls, which have been moving toward Trump, move back to Clinton, even before the DNC is completed. Still, it will be difficult to figure out if any trend away from Trump is really because of the Kaine announcement; there’s just so much going on right now. But if Trump’s bounce doesn’t begin to recede, then that probably means the Kaine announcement didn’t help and that should be worrisome to the Clinton campaign.

A woman president wouldn’t erase centuries of male-dominated politics

Harry Enten was a senior political writer and analyst for FiveThirtyEight.