PHILADELPHIA — Maybe the Big Dog has one last trick in him.  

Every four years for a generation, Democrats awaited Bill Clinton’s speech at the party’s national convention the way classical musical lovers anticipate the first draw of Yo-Yo Ma’s bow across the strings of a cello.  

They were certain they’d hear something spectacular — and familiar — but they weren’t sure what wrinkles and flourishes would punctuate the performance.  

Even President Barack Obama, the party’s best orator, has turned to Clinton as his “explainer in chief,” a role the former president played for the current one in eviscerating Mitt Romney and Paul Ryan during the 2012 Democratic convention.  

But there’s concern within party ranks here that when Clinton takes the stage Tuesday to endorse his wife, he could be heckled by Bernie Sanders’ supporters. Either way, Democrats say he no longer outshines the rest of the party’s constellation the way he once did.  

Some attribute that to him losing a step politically over time, others to the ascendance of Obama and Hillary Clinton as leaders of the party and still others to the All Star cast of speakers slated for this convention.  

“There are so many stars,” Rep. Elijah Cummings, D-Md., said in an interview.  

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The first night alone featured first lady Michelle Obama delivering a memorable endorsement of Clinton, and a trio of Senate stars who have captivated national Democrats: Bernie Sanders of Vermont, Cory Booker of New Jersey and Elizabeth Warren of Massachusetts. They all laid down pieces of the case for Hillary Clinton.  

But none of them are Bill Clinton in his prime. The question is whether he can recapture that old magic when it matters most for his wife.  

If past is prologue, he’ll have spent most of Tuesday writing his speech with a cluster of longtime advisers and won’t really be done with it until it’s delivered. Those who know him well say he’s still got it.  

“He is very deft at pointing out the shortcomings of the opposition with humor and intelligence, in a way that appeals to people across the political spectrum,” said one denizen of Clintonworld. “He can be especially persuasive with moderates in both parties and independent-minded people, rationally explaining the differences in making the case for why Hillary is best for America.”  

But there’s no question, Democrats say, that his role has been reduced — after all, he’s giving a speech on the second night of the convention, not taking one of the more coveted third- or fourth-night slots.  

Some Democrats believe he’s lost a step. Others think he ought to give a pedestrian speech so as not to remind voters that he — not Hillary Clinton — possesses the kind of charm that electrifies a room and permeates television screens at the same time. So, there’s a good reason they’re not giving back-to-back speeches at this convention or at campaign stops around the country.  

Bill Clinton’s charge is a unique one. It is to tell voters, in convincing terms, why they should see his wife’s capacity for the presidency the way that he does.  

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“He, like others, can continue to talk about the personal qualities that make her a great president — not only her intelligence and experience, but also her steadiness, determination, passion and compassion,” the Clintonworld veteran said.  

On a couple of levels, though, Clinton is the worst possible character witness. He’s biased for his wife and his middle name isn’t honesty — or anything like it.  

And yet there are parts of Hillary Clinton’s story that only her husband can tell from firsthand experience.  

“He can speak to how her desire to improve the lives and future of children has defined much of her work over the years and will define her presidency,” the Clintonworld source said.  

A little biography probably won’t hurt, but Bill Clinton’s message has to be about framing the contrast between Hillary and Donald Trump on her terms. He’s in an ironic spot: The better he does at campaigning for his wife, the more likely he is to become a historical footnote to the story of the first woman president.  

If Bill Clinton can manage to put his wife’s interests before his own on the stage tonight, the Big Dog will have learned a pretty valuable new trick.  

Roll Call columnist Jonathan Allen is co-author of the New York Times-bestselling Clinton biography “HRC” and has covered Congress, the White House and elections over the past 15 years.

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