Joe Biden’s poll numbers are crumbling in the early nominating states that matter most.

Once the dominant front-runner in the Democratic primary, Biden is now marginally trailing Elizabeth Warren in the first caucus state of Iowa and the first primary state of New Hampshire. His South Carolina firewall shows signs of cracking and he's losing his once-overwhelming lead in Florida, according to a raft of recent polling.

Biden’s descent has been months in the making, the result of continuous fire from progressives, questions about his age and stamina, a drumbeat of negative coverage over lackluster debate performances and frequent misstatements, according to pollsters and party insiders. They also point to a campaign message that at times over-emphasized attacking President Trump and his claim to be the “most electable” Democrat in the field.

But perhaps the biggest factor has been the rise of Warren, the Massachusetts senator who has served up a steady diet of grassroots outreach and in-depth policy proposals that have endeared her to progressives.

“Biden has a challenger now. He didn’t have one before,” said Ryan Tyson, a Florida-based pollster who shared three large surveys he just completed in New Hampshire, South Carolina and Florida that show Biden slipping and Warren gaining.

While national polls have shown less movement in the race, the state-based surveys provide a more useful indication of the trajectory of the primary.

“If you see Warren winning in Iowa and New Hampshire back to back, whoa! Geez! Biden starts losing his argument about electability,” Tyson, who typically surveys for Republicans, said. “Can Biden hang on to South Carolina if he loses in the other early states? I don’t know.”

The former vice president continues to lead most national polls. He’s run ahead of Trump in general election match-ups in every major poll conducted this year. But the downward trend in Biden’s primary election top-line numbers and favorability ratings — which began long before reports surfaced recently detailing how President Donald Trump pushed Ukraine to investigate old business ties involving the former vice president’s son -- suggests several bruising months have taken a toll.

“Biden’s support was always soft. That’s the key,” said Patrick Murray, director of the Monmouth University Polling Institute. “Unlike some of the other candidates, Biden’s support isn’t as locked in. He doesn’t have that ‘it’ factor.”

The Biden campaign, which has bristled both at media coverage of the candidate and the attention paid to polls, would not comment for this article.

With more than four months until Iowa’s Feb. 3 caucus, there is plenty of time for the dynamics of the race to change. But there’s also cause for some alarm for Biden. In New Hampshire, Tyson’s just-completed 600-likely voter poll shows Warren with 18 percent of the vote and Biden 15 percent in an open-ended ballot question. It’s a dramatic change from his last poll, with Biden dropping 18 points while Warren gained 7 — a 25-point shift.

While the methodologies differ slightly, those New Hampshire numbers resemble a Monmouth University poll released Tuesday, which had Warren leading Biden by 2 points in a survey of 401 voters.

“We are seeing in our poll that people are saying Warren is electable. She’s pragmatic,” said Murray. “I heard that when I talked to voters in Iowa and New Hampshire, and we’re seeing that in polls now.”

Similarly, since May Biden has dropped by another 18 points in South Carolina, though he still remains in first place there with 19 percent of the vote, according to Tyson’s 600-voter poll.

Tyson’s polls were conducted for a political nonprofit, Let’s Preserve the American Dream. It does not disclose its donors and has links to Florida business interests, but Tyson says it has also worked with Democratic-leaning as well as conservative groups.

Warren, who has spent relatively little time and money in South Carolina, has gained just a point since May and has 9 percent support in the poll. But she’s now in second because Bernie Sanders has tumbled there as he has in New Hampshire and Iowa.

Biden’s level of support in South Carolina makes it his firewall state, but even in South Carolina there are troubling signs of erosion. While he remains on top, among black voters, who are more than 60 percent of the Democratic electorate, Biden has plummeted 19 points in Tyson’s polls. That’s a potential leading indicator of the problems he could face after South Carolina’s Feb. 29 primary when many of the minority-heavy Southeastern states — as well as Texas and California — beginning voting on Super Tuesday, March 3, and thereafter.

Florida, where about 28 percent of the Democratic primary electorate is black, votes March 17. Biden is in first there with 24 percent of the Democratic vote, losing 15 points since May in Tyson’s polls. Warren moved into second with 11 percent, a 6-point increase while Sanders is in third with 5 percent, an 11-point loss since before the first candidate debate.

The percentage of Democratic voters who were undecided also shot up by double-digits in polls of the state.

In Iowa, Warren has pulled ahead of Joe Biden — marginally — for the first time, according to the latest Des Moines Register/CNN/Mediacom poll released Saturday. Pollster J. Ann Selzer’s highly regarded survey of caucus-goers showed Warren was benefiting from an enthusiasm gap — 32 percent said they’re "extremely enthusiastic" about caucusing for the Massachusetts senator, compared to 22 percent those who support the former vice president.

Selzer said that Warren’s “footprint” of voters — the percentage of people saying she’s a second choice or that they would consider caucusing for her — is bigger than Biden’s by 10 points.

Overall, Warren has 22 percent support to Biden’s 20 percent — a net 11-point shift in Warren’s favor since the last Selzer poll for the Des Moines Register in June.

“Biden’s favorability dropped, his unfavorable numbers doubled. In the absence of any context, it’s not bad. But Elizabeth Warren’s numbers are strikingly good,” Selzer said.

Selzer noted that Biden is “still a force” and that the poll she took was “worse for Bernie Sanders.”

Compared to the other early states, there’s a dearth in polling in third-in-the-nation Nevada. A survey released Tuesday from USA Today/Suffolk University showed Biden hanging on to a 23-19 percent lead over Warren, whose campaign in the state has earned high marks earned high marks from Democrats.

Nationally, Biden’s top-line numbers haven’t fallen as dramatically as in the early states. But pollster Peter Hart said his last national survey for the Wall Street Journal and NBC showed signs of weakness. In April, when Biden first officially entered the race, 32 percent said they were enthusiastic about backing him. In the last poll, 23 percent said that of Biden.

Hart said Biden could experience a rallying effect in recent days from Democrats, who are moving forward with impeachment plans over Trump's alleged threat to cut off U.S. military aid to Ukraine unless officials there launched an investigation into Biden and his son.

But beyond talking about Trump, Hart said, he hasn’t heard Biden emphasize policy the way Warren and others have. And that could leave voters wanting more.

“Instead of controlling the race, the race is controlling him,” Hart said. “The hardest thing to do is be a front-runner without an agenda when compared to the candidates to the left of him who are presenting bold plans that galvanize people where they say, ‘I want to be with Joe Biden.’”