When Bernie Sanders finally offers his endorsement of Hillary Clinton in Portsmouth, New Hampshire tomorrow, it will be the culmination of a month of aggressive courting by her, including a high-profile meeting in Washington and a dinner between their campaign managers in Vermont.

But much remains unknown about how - and whether - the political marriage being unveiled will actually work.

While they have a common enemy in Republican Donald Trump, Clinton and Sanders don't have much of a personal or professional relationship. And many of their supporters remain deeply suspicious of the other candidate.

Sanders diehards remain sceptical that Clinton, the favourite of the Democratic establishment, will embrace the agenda of a candidate who promised a political revolution. And Clinton boosters are wary of a longtime independent who questioned Clinton's judgment and was slow to accept defeat.


"I'm not convinced he's going to spend most of his time campaigning for Hillary Clinton and articulating her views," said Jim Manley, a longtime Democratic operative and Clinton backer.

"I'm concerned he'll continue talking about his views. ... Hope springs eternal, but I'm not exactly comforted by the actions I've seen over the last few weeks and months. He's been playing Jedi mind games. For every nice word he says here, he takes a couple other back there. Put me down as sceptical."

It also remains unclear how often - and under what circumstances - Clinton plans to deploy Sanders as a surrogate between now and November. As of today, no other appearances on her behalf had been nailed down, aides said.

Still, tomorrow's appearance will be a politically happy one for both Clinton and Sanders.

Sanders supporters say the past month has been about ensuring that he can make a credible case that his "revolution" will continue, though perhaps not at the same pace if he were the nominee. He can now promise, they say, that Clinton will carry the torch on key issues he championed during his surprisingly strong bid, including making college tuition free for many families and moving the country closer to universal healthcare.

"He kind of earned the right to take his time," said Congressman Raul Grijalva, one of Sanders's earliest congressional endorsers who is now backing Clinton. "It's good for Bernie that there was a time when people could celebrate what occurred."

Clinton, meanwhile, can head into the Democratic convention in Philadelphia able to project an image of party unity, as she stands arm in arm with her sometimes pesky primary rival.

Jeff Weaver, Sanders's campaign manager, credited the Clinton campaign for its willingness to incorporate many of Sanders's priorities into both her agenda and the Democratic platform that will be adopted at the convention later this month.

"What has been sent is a clear message that the voices of the 13 million-plus people who supported Senator Sanders are being heard," Weaver said, adding that Sanders got "way over 90 per cent of what we wanted" in the platform process, including a commitment to pursue a federal minimum of wage of US$15 an hour and bold actions on climate change.

"We have to keep moving the ball forward, and the ball will continue to move forward," Weaver said. "Electing Donald Trump would set that back tremendously."

Aides to both campaigns say a pivotal moment leading to tomorrow's announcement came nearly a month ago, when Clinton and Sanders agreed to meet behind closed doors at the Capitol Hilton in Washington on the night of the final Democratic primary, in the District. It remains their lone one-on-one meeting.

What soon became clear is that Sanders was focused on winning concessions on policy - and that Clinton was willing to accommodate him, at least to a point.

As soon as the principals departed that night, Weaver and Clinton campaign manager Robby Mook remained for two hours to continue discussing areas where the campaigns could work together.

In the weeks since, Mook and Weaver remained in near-daily contact by phone and text messages. Policy staff from both campaigns - as well as Sanders' wife, Jane - worked to craft proposals to advance Sanders's agenda but remain consistent with Clinton's principals.

Bernie doesn't always exude warm and fuzzy. He's not Mr Congeniality. That's not his appeal


Sanders, for example, had championed making college tuition free for everyone who attended public universities and colleges. Clinton often derided his proposal, saying taxpayers shouldn't foot the bill to send Donald Trump's kids to college. The compromise eventually crafted calls for free tuition for families making up to US$125,000 a year.

Underscoring the progress that was being made, Mook travelled to Vermont late last month for dinner with Weaver at the Farmhouse Tap & Grill in Burlington, the city where Sanders was once mayor and maintained his presidential campaign headquarters.

Last week, Clinton rolled out revamped proposals on college tuition and healthcare, promising to push for a "public option" that would allow people to buy into government insurance as part of the Affordable Care Act. That was a far cry from Sanders' proposal for a single-payer system but seemed a step in the right direction.

Next, during platform hearings in Orlando over the weekend, Clinton policy adviser Maya Harris and Sanders policy adviser Warren Gunnels sat side by side at the same table to hammer out compromise language on key positions.

Jesse Jackson, who similarly withheld an endorsement of the Democratic nominee when he ran for president in 1988, said he saw parallels with Sanders this year.

"What I chose to do was expand the Democratic Party, even though it did so kicking and screaming," Jackson said.

The process in 2016 has been markedly different than 2008, when a vanquished Clinton was much quicker to declare her public allegiance to then Senator Barack Obama.

Obama and Clinton met alone in 2008 days after the final primary in the living room of Senator Diane Feinstein. Clinton endorsed Obama shortly thereafter.

"Is it ever going to be as warm and fuzzy as it was eight years ago? I don't know," said Mo Elleithee, who was an aide to Clinton in 2008. "Bernie doesn't always exude warm and fuzzy. He's not Mr Congeniality. That's not his appeal."

There have been discussions between the two camps about deploying Sanders to states where he performed well in the primaries, such as Wisconsin and Michigan. New Hampshire also fits that definition: He beat Clinton there by 22 percentage points.

Another possibility is to send Sanders to college campuses to rally the youth vote, which sided with him by large margins over Clinton. There is a potential optics challenge, however, if thousands of Sanders fans, wearing "Bernie" T-shirts, come out because of their devotion to him and not the party's nominee.

For some in Clinton's camp, Sanders's endorsement has been viewed as more of a utilitarian exercise. His endorsement signals to progressive backers - including his largest union endorser and the Congressional Progressive Caucus PAC - that the time for unity has arrived.

But as Sanders waited, the value of his endorsement also waned, as a parade of other Democratic all-stars, including Obama, Vice-President Biden and Senator Elizabeth Warren hit the campaign trail for Clinton.

Sanders also faces the reality that some of his supporters will be deeply disappointed that he's endorsing Clinton.

Stacey Patel, 42, a management consultant and Sanders delegate from Florida, was hoping for a contested convention in Philadelphia - and said Sanders's appearance with Clinton will not guarantee that she will vote for Clinton.

"I think it will be difficult to see this," she said, "especially for low-income and young people who were looking forward to a contested convention, and have been doing car-washes and all these other things to pay for the trip. There's a lot of disappointment, and there's a lot of disenfranchisement, frankly."