WASHINGTON — This time, Bernie Sanders' presidential campaign has to be about Bernie Sanders — and that will be his greatest challenge.

When he decided to run for the Democratic nomination in 2016, the iconoclastic independent senator from Vermont told friends that he wanted to make sure a progressive agenda was front and center in the national political debate. If other progressives, such as Sen. Elizabeth Warren, D-Mass., were going to sit out the election and let Hillary Clinton tack to the center, he would jump in and pressure her from the left.

But the Democratic Party has changed — in no small measure because of Sanders' first campaign — and his message is no longer so distinctive. He is a victim of his own success in that the planks of his 2016 campaign have become liberal litmus tests that many of his rivals for the 2020 nomination are trying to pass: "Medicare for All" and free college tuition among them.

For Sanders to win, he'll have to convince Democratic primary voters that he's the right person for a very specific mission.

"They care more about beating Donald Trump than who the candidate is," said Bill Press, a liberal talk show host and former California Democratic Party chairman who backed Sanders in 2016 but hasn't yet picked a favorite for 2020. "It’s not going to come to him automatically. He’s going to have to fight for it."

That's an uncomfortable position for a politician who has spent his career playing down personality and using the ideological purity of his policy preferences as a political cudgel. The safe space for Sanders — who has few close friends in Congress after 30 years in the House and Senate — has always been to focus on everything but personal qualities.

But now it has to be about him — about why he's the one.

The crowded primary presents a related challenge for Sanders. In 2016, his policy stands allowed him to contrast with his main rival, Clinton, in ways that capitalized on the antipathy some Democrats had for her on a personal level. He cast himself as honest and authentic, not beholden to special interests and uncompromising on progressive priorities.

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This time around, it will be harder for him to distinguish himself against a variety of other candidates in the same way — and none of them have the depth or complexity of relationship with the Democratic electorate that Clinton did.

Already, Sanders is showing signs that he understands he has to adapt to thrive. In his campaign launch video, released early Tuesday morning, he put environmental, racial and social justice on par with the economic justice he has long prioritized as the pillars of his message. That's a sign that he's taken to heart the criticism that his heavy emphasis on economic issues — particularly those that animate working-class white voters — hurt his ability to connect with people of color.

Perhaps more importantly, as he tries to distinguish himself in a field full of candidates echoing his positions or carefully calibrating theirs in close proximity to his, Sanders reminded his supporters that he's the original, not a watered-down imitation of the political revolutionary they fell in love with.

"Together, you and I and our 2016 campaign began the political revolution," he said. "Now, it is time to complete that revolution and implement the vision that we fought for."

That is, he has no plans to watch from the sidelines as someone else tries to lead Democrats to the Promised Land.

A Sanders campaign spokeswoman did not reply to a request for an interview for this piece.

Still, Sanders comes into the race, like any other repeat candidate, with both experience and baggage. It won't be lost on anyone that no matter how much his campaign tilted the debate in the Democratic Party afterward, he lost the 2016 primary to Clinton — and many of her supporters still blame him, fairly or not, for a lack of enthusiasm among his supporters for her general election campaign.

But Sanders, who called Tuesday for his backers to create a force of a million volunteers, has a national network of small-dollar donors — the lifeblood of modern Democratic campaigns — a bumper crop of young political activists and elected officials who have cast themselves in his image and an outside group, called "Our Revolution," that remained active in his stead during last year's midterm elections.

Sanders and his team of political operatives also have the campaign experience that they lacked last time, which means they should have a better appreciation of the mechanics of putting together the state-by-state operations necessary to win delegates to the Democratic convention.

And, under new party rules, the Democratic "superdelegates" — a cadre of party officials who went almost unanimously for Clinton in 2016 and were almost uniformly hostile to Sanders because he was not of the party — will not come into play unless no candidate wins the nomination on the first ballot.

It will be almost a year before the Iowa caucuses start producing winners and losers, but, Sanders' entry feels like a big victory for the left, according to Adam Green, co-founder of the Progressive Change Campaign Committee.

Though the PCCC has endorsed Warren, Green said he sees the two progressive senators as a "one-two punch" that will keep the primary debate focused on issues of core concern to the left.

"It really matters in terms of what issues get the spotlight in the 12 sanctioned debates and the day-to-day debate in the press," he said. "As there’s a rush of candidates to match Sanders and Warren on the issues, that inherently changes voters' perception of what ideas are in the mainstream and what ideas are viable against Trump."

But it's not just about the message anymore.

"We hear from a lot of our own members that the No. 1 thing they want is to beat Trump," he said. If all goes well for the PCCC, Sanders and Warren will create a galvanizing force for the ideas of the left and ultimately produce a progressive nominee.

"We believe Elizabeth Warren is the most electable against Trump," Green said.

That's the rub for Sanders, whether it's against Warren or other rivals: He has to make the case he's the best champion for the cause of winning the White House.