Sen. Bernie Sanders (I-Vt.) is off to a splashy start in his second run for president, quickly trumpeting a massive fundraising haul in his first 24 hours as a candidate, luring millions to his Tuesday announcement video and brawling with President Trump — signs of his potential to become a major factor in the Democratic primary.

The initial surge and one-day receipts of $6 million reflect a resilience of support for the Democratic runner-up in 2016 and served notice to his competitors, who have so far been unable to create the same groundswell with their campaign launches.

It also reinforced the strength of his established political base at a moment when other candidates are only beginning to introduce themselves to voters. That is an asset that could prove valuable, should the contest splinter the party into many different pieces.

But Sanders, 77, also is confronting a radically different political landscape than the one he faced four years ago and potential obstacles toward regaining the surprising strength he showed in 2016, when he benefited from being the only serious opponent to the eventual Democratic nominee Hillary Clinton.

This time, he is facing a crowded and diverse field of challengers who, if not able to match Sanders’s fundraising prowess, have demonstrated some financial strength and have played to overflow crowds in the early voting states. Some of them are doing so with the help of staffers and supporters who backed Sanders in 2016.

Sanders, for his part, has yet to prove he can expand his appeal past his limited, if loyal following of younger voters, mostly young men. He had particular issues in 2016 in attracting older women and nonwhite voters, two Democratic blocs whose electoral prominence has only grown in the past two years.

Beyond those hurdles, Sanders has faced scrutiny over the way his campaign team handled sexual misconduct claims in 2016 and, perhaps most challenging, is no longer the sole advocate in the race for liberal priorities such Medicare-for-all, a higher federal minimum wage, free community college and other policies embraced by many in the field.

Nevertheless, the first day of Sanders’s second campaign swiftly demonstrated that he still commands an audience of both defenders and detractors, surprising some Democrats who have long expected him to run but were unsure how potent a force he would prove to be.

“This is truly remarkable,” President Obama’s reelection campaign finance director, Rufus Gifford, tweeted Wednesday about Sanders’s first-day haul. “I was skeptical of his ability to match his 2016 grassroots $$ magic. I was wrong.”

Democratic strategist Jon Reinish, who is not affiliated with any 2020 campaign, said there was “no question” that Sanders “has had an impressive 24 hours.”

But, Reinish said, Sanders also “enters the race with some significant baggage.”

Sanders said Wednesday that his $6 million in donations within a day of his announcement came from 225,000 contributors.

By comparison, Sen. Kamala D. Harris (D-Calif.), a rival candidate seen by some as having had the best previous leap into the race, said she hauled in about $1.5 million in her first 24 hours.

Sanders also will be able to transfer some of the more than $9 million in his Senate campaign account at the end of 2018, according to federal campaign finance records.

Small-dollar fundraising, the type that benefited Sanders in 2016 and this week, could emerge as a critical factor in the Democratic primary. There is growing skepticism among Democratic voters about wealthy benefactors’ influence on the political process, posing risks for candidates relying on large donations. Sanders ranks as the best example of that; in 2016 he kept his campaign in the running until the end of the primaries, although Clinton had a lock on the party’s major donors.

The surest sign of Sanders’s standing in the Democratic field may have come from the president the Democratic candidates are seeking to defeat. President Trump appeared to be trying to elevate the firebrand liberal as part of a broader attempt to cast Democratic candidates as far-left extremists, as he escalates his effort to win a second term.

“Crazy Bernie has just entered the race. I wish him well!” Trump tweeted Wednesday morning. On Tuesday, a Trump campaign spokeswoman issued a statement seeking to tether other candidates to him.

A couple of hours later, Sanders responded with his own social media missive and a link to a page on his website where people can provide their contact information.

“What’s crazy is that we have a president who is a racist, a sexist, a xenophobe and a fraud. We are going to bring people together and not only defeat Trump but transform the economic and political life of this country,” Sanders tweeted, words that echoed his announcement.

Sanders, who identifies as a democratic socialist, is signaling that he plans to run a campaign reprising many of the signature themes he highlighted in 2016: Medicare-for-all, combating income inequality and aggressively addressing climate change, among other things.

“He will run an issue-focused campaign,” said Sanders adviser Ari Rabin-Havt, who also noted that Sanders’s leadership on major issues will be part of the case he makes to voters.

The Vermont senator’s two-minute Tuesday launch video, which had been viewed more than 6 million times on Twitter as of Wednesday afternoon, emphasized that Sanders has been at the forefront of the fight for liberal ideas that have become focal points in the contest.

Much of the rest of the field has gravitated to the positions that Sanders has advocated. While some liberal activists see that as a positive development, it could also complicate Sanders’s path to the nomination, leaving voters with a potential choice between the first advocate or one who shares Sanders’s views but may seem more appealing or electable.

Over Presidents’ Day weekend in New Hampshire, a key early state whose primary Sanders won in 2016, Sens. Harris and Cory Booker (D-N.J.) both noted their support for Medicare-for-all, though the latter pointed out the difficulty of passing it.

Both candidates drew large and enthusiastic crowds. At a church in Portsmouth on a snowy afternoon, Harris drew 1,000 people inside, according to her campaign’s estimate, with 500 or so outside. A house party for Booker in Manchester was packed with interested Democrats eager to hear him speak.

But the more intense competition ahead could be between Sanders and Sen. Elizabeth Warren (D-Mass.), another favorite on the left with a similar message of taking on the wealthy and powerful and being a champion for the working class.

“Bernie getting in will draw some support in the short term from Warren and also several other candidates,” said Adam Green, co-founder of the Progressive Change Campaign Committee, which has endorsed Warren. Green also predicted Warren and Sanders would become a “one-two punch that drives the debate among the entire field.”

For Sanders, rival candidates are not the only thing he will have to worry about. After his struggle to win over African American voters in 2016, he now faces a field with at least two black candidates, Booker and Harris.

In the matter of appealing to women, Sanders publicly apologized last month to women who felt mistreated by his campaign staff members in 2016. Sanders has not clarified, however, when he knew of the allegations and what role he had in adjudicating complaints.

In one notable change from 2016, Sanders has a largely new campaign team. He has hired Faiz Shakir, a veteran political operative who had been serving as the national political director for the American Civil Liberties Union, to manage his 2020 campaign.

In the past, Shakir worked for former senator Harry M. Reid (D-Nev.) and Rep. Nancy Pelosi (D-Calif.), the speaker of the House.

Gilda Cobb-Hunter (D), president of the National Black Caucus of State Legislators and a veteran South Carolina lawmaker, said Sanders “seems to have learned the lesson from before — he is doing outreach in communities of color and recognizing the need to do that.”

“It seems to have dawned on him that he is not the only game in town, and as a result I see that he is more aggressive in his effort here,” she said of her home state. “I do think he had not much choice but to get out of the gate fast. This is a totally different field than when he ran before.”

John Wagner contributed to this report.