What an Obama endorsement will mean for Hillary

Barack Obama is about to tell Bernie Sanders that the revolution stops now.

On Tuesday evening Mr Obama called Hillary Clinton to congratulate her on securing enough delegates to "clinch" the Democratic nomination.

He also called Bernie Sanders to thank him for his hard-fought campaign. At the Vermont senator's request, the two will have a meeting on Thursday to discuss "how to build on the extraordinary work he has done to engage millions of Democratic voters", in the words of a White House press release.

In other words, it's time to wrap things up and unite the party behind Mrs Clinton. The president is the leader of the Democratic Party, and it's his legacy on the line. Continued acrimony within the ranks will only complicate matters.

Now it's only a matter of time before Mr Obama formally endorses Mrs Clinton and hits the campaign trail to support her.

He's reportedly itching to enter the political fray one more time and take aim at Donald Trump, who he sees as disparaging the coalition of voters that propelled him to the presidency.

So when Mr Obama endorses Mrs Clinton, how much of a boost will it give his former secretary of state? And will it come in the right places?

Here's a look at where Mrs Clinton needs the most help - and what Mr Obama's endorsement could do for her.

Progressives

The success of Bernie Sanders's insurgent, anti-establishment primary campaign has shown that Mrs Clinton has weakness on her left flank.

For instance, in New York - a state Mrs Clinton won convincingly - Mr Sanders carried "very liberal" voters 56% to 44% in exit polls. National polls have tracked close to that margin, as well.

Mr Sanders has won the hearts of many a liberal by promising free college education, single-payer healthcare and Wall Street reform, and he's condemned international trade deals. Mrs Clinton was not nearly as ambitious in her proposed agenda - instead stressing incremental advances - and her supporters were seldom as enthusiastic.

Mr Obama defeated Mrs Clinton in the 2008 Democratic primaries in part by rallying the most liberal voters to his side. Since then, however, some of those supporters - the ones who have since moved into Mr Sanders' camp - have expressed disillusionment with his inability to enact a sweeping progressive agenda.

The president, however, is still extremely popular among all Democrats, including liberals. When he talks, they'll listen.

Will he help? Yes. But the endorsement of someone like Senator Elizabeth Warren, a progressive heroine, is what could really carry the day.

Youth

Young voters were a key part of the coalition that not only helped Mr Obama win the Democratic nomination in 2008 but also powered him to two general election victories.

Against Mitt Romney in 2012, Mr Obama carried 67% of the vote. The 18-to-29 age group still gives the president some of his highest approval ratings.

This year young voters have been providing Mr Sanders with eye-popping margins. In the 2008 primaries Mr Obama won 60% of the under-30 vote. This year the Vermont senator has carried fully 71% of that age group - making him, according to Princeton Prof Matt Karp, "probably the most popular young people's candidate in US history".

Young people can be very suspicious of attempts to move their opinion when it comes to commercial advertising, so Mr Obama's endorsement will have to carefully crafted to avoid seeming like a sales pitch. Given his track record, however, he should be able to pull it off.

Will he help? Mrs Clinton needs all the Obama cool that he can offer.

Minorities

Blacks and Hispanics were another key portion of Mr Obama's winning coalition, and Mrs Clinton will need robust turnout from these voters again - particularly in key electoral battlegrounds like Florida, North Carolina and Virginia - if she wants to replicate the president's success.

Unlike the previous two categories, minority voters have been a source of strength for Mrs Clinton throughout the primaries. The black vote in particular propelled her to massive wins across the South that gave her a lead over Mr Sanders that she never relinquished.

Where Mr Obama's support will help, however, is in turnout. Black voters set a record high turnout in 2008 at 69%. In 2012 it dropped to 67%, but that was notable as well because for the first time in US history black turnout surpassed that of whites.

For Hispanics, Mr Obama will be more of a mixed bag. While he has won praise for his unilateral action on immigration reform, Hispanic activists have noted that he prioritised other issues when he had congressional majorities early in his term - and his administration has set a record for total number of deportations.

Fortunately for Mrs Clinton, her autumn opponent is Donald Trump - who has his own image problems among this rapidly growing portion of the electorate.

Will he help? Mr Obama may not be able to deliver the kind of historic numbers that turned out in 2008, but he's still a force to be reckoned with.

Working-class whites

Mr Obama struggled with white, working-class voters in 2008 even before he made his famous off-the-cuff comments about how they "cling to guns or religion or antipathy to people who aren't like them".

During the 2008 primary season, Mrs Clinton trounced Mr Obama among whites - particularly in the South - winning by 49% in Kentucky, 47% in Alabama, and 44% in Mississippi. In the 2012 general election, Mr Obama carried only 36% of the white vote.

It seems likely that the key battleground in the forthcoming Trump-Clinton general election will be the industrial Mid-West and Pennsylvania, where the working-class white vote could prove instrumental.

It's no coincidence that the first two campaign stops for Mrs Clinton after securing the Democratic nomination this week are Ohio and Pennsylvania.

Will he help? This isn't where Mr Obama will make his mark.

A bit of history

Mrs Clinton finds herself in an unusual spot. She has an incumbent president who is both well liked - with approval ratings climbing over 50% - and free from political baggage.

George W Bush had an approval rating in the 20s during the 2008 presidential campaign. He made few appearances on nominee John McCain's behalf and only spoke by video at the Republican National Convention.

In 2000 Democrat Bill Clinton was popular - but his impeachment during the Monica Lewinsky scandal made him damaged goods politically. Even though he endorsed Al Gore in the Democratic primary race against Bill Bradley, the then-vice-president chose to distance himself from Mr Clinton on the campaign trail. His selection of Connecticut Senator Joe Lieberman, who had been a critic of Mr Clinton's sex scandal, was seen as a rebuke of the incumbent.

It requires going back all the way to 1988 to find the last time an incumbent president was an unalloyed good in a presidential election. Ronald Reagan, despite the Iran-Contra scandal several years earlier, was still very popular and an active presence on the campaign trail. He gave George HW Bush a decided boost during the Republican convention, positioning his vice-president as the heir to his political legacy.

And before that? 1968 wasn't exactly a good year for incumbent Lyndon Johnson, with Vietnam roiling the nation.

In 1960, when Dwight Eisenhower was asked for an example of a major idea his vice-president and Republican nominee Richard Nixon had contributed during his presidency, he said: "If you give me a week, I might think of one".

Mrs Clinton must hope she gets a more ringing endorsement than that from Mr Obama.