CHART: How Obama's Approval Compares With Past Presidential Convention Speakers

President Obama will make the case for Hillary Clinton Wednesday night with about as many Americans approving of him as disapprove of him.

That puts him somewhere in the middle of other outgoing presidents who have given convention speeches supporting their potential successors. Bill Clinton, Ronald Reagan and Dwight Eisenhower were all relatively well liked when they left office. George W. Bush and Harry Truman, meanwhile, delivered their addresses even while their approval numbers were in the tank.

Outgoing two-term presidents don't often have big roles in their successors' campaigns — it hasn't happened for more than a century, as NPR's Domenico Montanaro has pointed out.

Sometimes that president's low approval plays into how much a president campaigns — John McCain's campaign didn't involve the unpopular George W. Bush much on the campaign trail.

But Bush still spoke at the 2008 Republican convention (though he wasn't there; his speech was piped in via satellite). That happened despite some Republicans who thought he should keep out of the campaign altogether. But Bush spoke at the convention because still other Republicans really did like him, as Bush's former Office of Management and Budget Chairman (and now-Sen.) Rob Portman told the New York Times.

"The president's approval rating among Republicans' base voters who are needed for a successful McCain campaign is relatively high," he said then.

That meant the McCain campaign was trying to walk a thin line, as the Times' Sheryl Gay Stolberg wrote at the time — not aligning McCain too closely with Bush, but also not pushing Bush (and his base) away.

Meanwhile, it's not that a superpopular president comes without risks. As NPR reported during Al Gore's nominating convention in 2000, "many were hopeful going into the convention that [Bill] Clinton, the more accomplished orator of the two, would not deliver a speech that would overshadow Gore's own acceptance speech Thursday night."

Obama occupies the middle ground: He isn't as deeply unpopular as George W. Bush in 2008 or as Harry Truman in 1952, nor is he Bill Clinton 2000 or Ronald Reagan 1988.

One of his roles Wednesday night will be the role that many other speakers here in Philadelphia have tried to play: to create more party unity. And though Obama isn't broadly popular, Democrats love him — his approval rating within his party is in the upper 80s. When Obama's photo briefly came on in the Philadelphia arena on Tuesday night — one in a long string of presidential photos — the crowd broke into thunderous cheers.

Obama largely unites Democrats — that may mean he could reach Sanders supporters who are reluctant to support Hillary Clinton in November.

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