Sen. Bernie Sanders caught the establishment Democratic Party off guard three years ago; few anticipated the strength of his candidacy and message.

Things have changed. Sanders announced on Tuesday he is running for president again — but this time, he’s not the underdog. This time, he is a frontrunner who, despite coming in second in 2016, has fundamentally altered the Democratic Party.

Sanders, now 77 years old, is the most popular senator among constituents in America and consistently ranks among the top potential 2020 candidates in early polls. That said, he’s joining a packed field, including progressive firebrands like Sens. Elizabeth Warren (MA) and Kamala Harris (CA), and possibly Sherrod Brown (OH). Prominent Democrats such as Joe Biden are also expected to jump into the race.

Sanders, in an email to supporters on Tuesday, laid out a vision featuring a number of his longstanding policy priorities including free college and Medicare-for-all. “I’m running for president because a great nation is judged not by how many billionaires and nuclear weapons it has, but by how it treats the most vulnerable — the elderly, the children, our veterans, the sick and the poor,” the email reads.

As part of the message, Sanders is calling on 1 million supporters to sign on to his campaign and demonstrate the impact of grassroots backing — a key component of his 2016 presidential campaign that he intends to build on this cycle.

“We’re gonna win,” he said in an interview with CBS’s This Morning. “We are gonna also launch what I think is unprecedented in modern American history, and that is a grassroots movement to lay the groundwork for transforming the economic and political life of this country. That’s what’s different.”

I'm running for president. I am asking you to join me today as part of an unprecedented and historic grassroots campaign that will begin with at least 1 million people from across the country. Say you're in: https://t.co/KOTx0WZqRf pic.twitter.com/T1TLH0rm26 — Bernie Sanders (@BernieSanders) February 19, 2019

Sanders, now a well-established contender, faces a fresh challenge during his second presidential run.

The rallying cries that distinguished Sanders from Hillary Clinton in 2016 — Medicare-for-all, a $15 minimum wage, campaign finance reform — are no longer unique positions. In some ways, that’s an advantage for him; he’s become a progressive father figure who can credibly claim a lifelong dedication to the issues the party is now standing behind.

“I don’t think anyone thought you could run a presidential campaign without taking money from corporate PACs,” said Heather Gautney, a former Sanders policy adviser and the executive director at Our Revolution, a progressive activist group inspired by Sanders. “People who are running on Medicare-for-all or adopting these campaign finance ethics — all of that came out of the Sanders campaign.”

Sanders offers a nod to this in his 2020 launch message, highlighting just how effectively his 2016 campaign has helped spread a wide range of progressive positions. “Together, you and I and our 2016 campaign began the political revolution. Now, it is time to complete that revolution and implement the vision that we fought for,” he writes.

“I don’t want to say [the party came my way], but most people would say that,” Sanders told CBS.

On the other hand, Sanders has to make the case that he’s still the progressive that voters should choose. He’s remained consistent; he’s fighting for the same things today that he was in 2016. The question is: Can he win at his own game?

Sanders has started a movement Democrats can’t ignore

In 2016, Sanders said he would start a “political revolution.” Today, it’s hard to overstate the influence his campaign — which railed against corporate greed, economic inequality, and money’s influence in politics — has had on Democratic politics.

Sanders’s policy ideas have arguably had more of a lasting influence on Democratic politics than Clinton’s platform did. The policies Sanders campaigned on three years ago are now mainstream.

Sanders “is not a candidate who is adopting these issues — he has always been running on these issues,” said Joshua Ulibarri, a partner with the Democratic polling firm Lake Research Partners.

Sanders popularized single-payer health care with his Medicare-for-all bill. It’s now one of the most prominent progressive health care policies and increasingly popular among Americans — so much so that many House Democratic candidates campaigned on some variation of it in 2018. Medicare-for-all has come to mean many things, from a public option to getting rid of private health insurance altogether.

Sanders has been clear with his proposal: a bill that transitions the country to a universal Medicare system over four years, eventually sunsetting Medicaid and Medicare in their current forms, while leaving the Veterans Affairs health system and the Indian Health Services in place.

He normalized a $15 minimum wage. During the Obama years, the Democratic platform supported increasing the minimum wage from $7.25 and tying it to inflation. Clinton initially supported an increase to $12 per hour. Sanders, however, made the case for $15 an hour. At the time, it was a huge debate within the Democratic Party, but by the summer of 2016, Clinton had endorsed Sanders’s view and Democrats adopted it in their platform. Now the debate feels almost passé.

There has been a tangible shift toward class warfare in Democratic politics. The “millionaires and billionaires” that Sanders targets in every stump speech are now the basis of almost every leading progressive economic policy proposal. Especially in light of Republicans’ tax cuts, which gave the biggest windfall to corporations and wealthy Americans, Democrats have crafted a message around wealth distribution.

The issue has taken center stage in the 2020 battle of ideas as well. Warren unveiled her wealth tax on Americans with assets worth more than $50 million. Sanders, who in 2016 proposed a 65 percent top estate tax rate, this year has proposed raising that top rate to 77 percent.

And, of course, his calls for campaign finance reform. By the end of the last presidential primary, the country was well aware that Sanders’s average campaign donation was $27. Running without corporate PAC donations is now something politicians — especially 2020 contenders — now all feel pressure to conform to, and that’s largely because of Sanders.

In fact, a lot of these ideas have adopted, or are being echoed, by other 2020 contenders. Harris and expected candidate Sen. Cory Booker (D-NJ) have signed on to Sanders’s Medicare-for-all bill. All the top candidates support a $15 minimum wage. The list goes on.

Rebecca Lynch — a state organizer for Working Families Party, which endorsed Sanders early — said that when Sanders won the 2016 Democratic primary in Wisconsin, something shifted with state lawmakers. In the past, they wouldn’t have touched ideas like Medicare-for-all. Now they are all rallying around a state public health care expansion bill called “BadgerCare for all.”

“There’s huge push right now to expand BadgerCare,” Lynch said. “I don’t think that’s something that Democratic leaders would have supported [that] had it not been for Sen. Sanders.”

The feeling is echoed among left-wing grassroots groups as well. “This progressive movement feels like it can win elections,” Gautney said.

Sanders has to show he can grow his base — but not in the way you would think

The runup to Sanders’s presidential announcement wasn’t smooth.

Reports of gender and pay discrimination in his 2016 campaign and allegations of sexual harassment against some of his campaign surrogates have made for a rocky rollout of his 2020 campaign. When first confronted with the allegations, Sanders said he didn’t know of the complaints.

“I was a little bit busy running around the country trying to make the case,” he told Anderson Cooper, saying the campaign he ran in 2016 wasn’t perfect. Since then, he has delivered a more formal apology on the matter.

“To the women in our campaign who were harassed or mistreated, I apologize,” Sanders said. “Our standards, our procedures, our safeguards, were clearly inadequate.”

The complaints elevated a trope that Sanders’s campaign was propped up by the “Bernie Bro” — an army of young, mostly white men — who didn’t get him over the finish line. He got trounced by Clinton in Southern states, where African-American leaders said the Vermont senator just couldn’t connect.

“I do believe Bernie Sanders struggled then and clearly now with black voters overall,” Ray McKinnon, a black pastor from Charlotte, North Carolina, who is a member of the Democratic National Committee and former Sanders delegate in 2016, told Vox’s Ella Nilsen last year. “The guy’s from Vermont, it’s not like he has a massive [black] constituency. He failed to connect on a visceral sense.”

But recent polling tells a different story about his base. According to a December Quinnipiac University national poll, Sanders was more popular with women than with men, and had more support among African Americans and Latinos than whites. And he’s still leaps and bounds more popular than President Trump, who lands among the most disliked presidents in modern history.

“Bernie still has the biggest platform out of anyone in this race right now — outside of potentially Joe Biden,” Ulibarri said. “He has the biggest network. He has as big an echo chamber as anyone in this country. Bigger than Biden. Bigger than Harris. The question for Sanders is where else can he go? Does he have to go anywhere else?”