The fates of Hillary Clinton and Barack Obama, two former bitter rivals, are now inextricably bound together. Never will that be more clear than on Tuesday, when they campaign together for the first time in eight years in Charlotte, North Carolina.

Obama’s approval ratings -- and his Department of Justice, which, through the FBI, is investigating Clinton’s use of a private email server as Secretary of State -- will help determine if Clinton can become only the second person since World War II to earn a third White House term for the same party. Meanwhile, Obama needs Clinton to defend his legacy by defeating Donald Trump, who rose to political prominence waging a “birther” crusade against Obama and has vowed to undo many of the president’s accomplishments in office.

The Clinton-Obama relationship will face an even more immediate and dramatic test in the ongoing FBI investigation. On Saturday, agents spent more than three hours interviewing Clinton, who suffers from poor ratings on trust and honesty.

Obama has repeatedly defended Clinton on the issue, saying in April, “I continue to believe that she has not jeopardized America’s national security.” Comments like that, in addition to Bill Clinton’s recent impromptu meeting with Attorney General Loretta Lynch, have led critics to accuse the president of biasing the investigation being conducted by agencies he oversees.

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Obama remained formally neutral throughout the Democratic primary, but was a powerful asset for Clinton even in absentia.

Her tenure in his cabinet served as a tacit endorsement, which Clinton wielded effectively against rival Bernie Sanders. “I went with Barack Obama. You did not,” Clinton said during their final debate in Brooklyn, while knocking Sanders for voting against the auto bailout Obama championed.

Clinton narrowly lost the 2008 Democratic nomination to Obama in a race for the ages. She began her second presidential bid by saying at almost every campaign stop that she was not running for a third term of Barack Obama or her husband, former president Bill Clinton. But she quietly jettisoned that line at the beginning of this year.

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On Tuesday, the president can help Clinton pursue her two biggest goals of the moment: Win over Bernie Sanders supporters and paint Trump as patently unqualified to be president.

Obama’s job approval rating has ticked up to a healthy 50 percent, making him far more popular than George W. Bush was at this point in his presidency.

And Obama is popular with virtually every Democratic constituency in the country. Eighty-two percent of Sanders voters approve of the job Obama is doing, according to a recent NBC News/Wall Street Journal poll, along with 64 percent of young people, Clinton’s major demographic weak spot.

NBC News exit polls over the course of the primaries showed that most Democrats -- 53 percent -- want to continue Obama’s policies, while only 30 percent want the next president to be more liberal.

Meanwhile, Obama is also reasonably popular with more up-for-grabs groups, like suburban women, whom Clinton’s campaign is heavily courting.

Drawing on their bitter contest from eight years ago, Obama will likely make a direct appeal to Sanders supporters. He can say that he, too, didn't like Clinton initially, but has since come to understand why she’d be a good president and they should too.

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Eight years at around this time, Clinton and Obama made their first joint appearance together after the primaries in the symbolically chosen town of Unity, New Hampshire. Sanders has shown no interest in making a similar gesture, forcing Clinton to appeal to his supporters herself and with surrogates like Obama.

And the Clinton campaign believes there is no more compelling messenger than the president for its central critique of Trump, which is that he’s unfit to be commander in chief. Obama can draw on his eight years in the Situation Room, including four with Clinton, to vouch for her readiness.

There are only five people on the planet who know what it’s like to be president. Two of them—Obama, Bill Clinton (Jimmy Carter has sat out 2016 so far) -- are supporting Clinton, while the other two -- George W. and H.W. Bush -- are conspicuously not yet supporting Trump.

Vice President Joe Biden will campaign with Clinton in Pennsylvania on Wednesday.

The choice of North Carolina for Obama’s first outing with Clinton was not an accident. The campaign and the White House had originally planned to make the debut in Wisconsin, but rescheduled after the Orlando nightclub shooting. Switching from a Democratic-leaning swing state to a Republican-leaning one projects confidence, moving from defense to offense.

Obama narrowly won North Carolina in 2008, but lost it in 2012. Clinton aides believe it’s impossible for Trump to get the 270 electoral votes he needs to win the presidency without winning the North Carolina, and suggest Trump is behind is ground organizing. Clinton’s team deployed their first staffers to the state two months ago and has already started running TV ads.

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Clinton has broken with Obama on only a handful of issues, the largest of which is the Trans Pacific Partnership trade deal. But mostly she’s campaigned on preserving and expanding Obama’s legacy.

Republican attacks on Clinton as a third term of Obama have lost a bit of their bite as his job rating climbed. But there’s no guarantee it will stay high. A downturn in the economy, terror attack, or scandal could undermine Americans’ views of the current occupant of the White House, and thus hurt his favored heir.

And campaigning on continuity puts Clinton the uncomfortable position of defending the status quo in a year when many voters want to blow it up.