The campaign for the Democratic presidential nomination appears to have opened up, with 100 days to go before the Iowa caucuses kick off voting. There are new opportunities for some second-tier candidates – especially South Bend, Ind., Mayor Pete Buttigieg and Sen. Amy Klobuchar of Minnesota.

Both Buttigieg and Klobuchar used this month’s televised debate to hone their appeal to the moderate wing of the Democratic Party – directly by attacking Sen. Elizabeth Warren’s liberal policy proposals (especially “Medicare-for-all”) and implicitly by being younger than former Vice President Joe Biden.

Going into the debate, the emerging conventional wisdom appeared to be that – for all the energy of over 20 starting candidates – the race was quickly turning into a two-person contest between Warren and Biden.

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Warren was running a pitch-perfect effort and consolidating the support of the liberal wing of the party, which seeks fundamental change in the U.S. economic system.

Biden was riding a tide of polling that suggested he would be the strongest general election candidate. He was appealing to a party electorate whose anger at President Trump motivates many of them to care primarily about who has the best chance of achieving their top objective: defeating Trump.

But now folks are realizing the historical truism of the nominating process: It starts in Iowa, New Hampshire and South Carolina – and results there can often depart from national polling and have an impact on subsequent contests.

The first three contests rely more on in-state retail politics than on national support, and the one-state nature of each contest enables more-focused media campaigns.

The truism comes out in the polling. In the RealClear Politics averages, Buttigieg currently garners roughly twice as much support in Iowa and New Hampshire as he does nationally.

By contrast, Biden does slightly worse in those states than he’s doing nationally, while the other two leading candidates – Warren and Sen. Bernie Sanders of Vermont – run basically in line with their national performance.

The most recent USA Today Poll showed the leading “candidate” – at 29 percent – is “Undecided,” scoring over 10 points ahead of supposed front-runners Biden and Warren.

Other lower-tier candidates may also benefit from the one-state focus of the first three contests.

Klobuchar, from neighboring Minnesota, is mounting a full-court press in Iowa, vowing to visit all 99 counties. She is shuttling between there and New Hampshire.

But it’s not just Midwestern moderates Buttigieg and Klobuchar who are making strong bids in the early states.

Even novelty candidates – like Andrew Yang, who cleverly uses social media to lead his #YangGang, and self-funding billionaire Tom Steyer – are able to make a push in the smaller early states.

Steyer, though he has yet to articulate a real rationale for his candidacy, is funding enormous media buys in the early states – and as a result is registering 2 or 3 percent in polling there, while barely garnering 1 percent support nationally.

But in a wide-open, in-state race, it is even possible that some of the third-tier, but more traditional candidates – such as Sens. Cory Booker of New Jersey, Kamala Harris of California and Michael Bennet of Colorado, or Montana Gov. Steve Bullock – might figure out a way to break through or reanimate their fading campaigns.

Still, the three currently leading candidates – Biden, Warren and Sanders – retain the best chance of winning the nomination, even though the race has opened up, and they’re facing clear headwinds.

Warren has run into challenges from two sources. She’d been hoping that Sanders – currently third and, since May, consistently running at around 16 percent in national polls – would fade and enable her to consolidate the support of the left wing of the party, but that hasn’t happened.

Despite recent punditry saying Sanders was losing ground, he reported raising the most money of any candidate in the last reporting period, put in a solid debate performance not long after a heart attack, and won the support of Reps. Alexandria Ocasio-Cortez, D-N.Y., and Ilhan Omar, D-Minn.

The endorsements may not affect the bulk of the Democratic electorate, but show that Sanders still has a solid appeal to a significant segment of the left-wing of the Democratic Party.

While Warren can turn that lemon into lemonade by saying she’s more moderate than Republicans like to portray her, she ran into other challenges in the debate. She wasn’t able to articulate a good response to questions about whether “Medicare-for-all” will result in higher taxes.

Face it: “Medicare-for-all” will require tax increases, but one can argue that middle-class Americans will pay less with government-funded health insurance than they pay today separately in both taxes and for private health insurance premiums.

Similarly, Biden has run into headwinds despite a political environment that appears tailor-made for him. With an impeachment inquiry surrounding claims that President Trump sought to extort Ukraine to provide dirt against Biden, it ought to be easy for Biden to make the case that he’s the Democrat Trump fears the most.

It also ought to be easy for Biden to inoculate himself against questions of whether he helped his son get a high-paid position on the board of a Ukrainian company, but it hasn’t been. Instead, Biden seems to have trouble getting those questions behind him. Every time I watch Biden, I think: Presidents Reagan, Clinton, or Bush would’ve had enough political jiu-jitsu to turn this Ukraine story into a winner. Why can’t you?

My sense is that the troubles for Warren and Biden are not over. If the two continue to face challenges in consolidating their separate wings of the party, other candidates may soon claim their place in the spotlight, and could catapult themselves into a leading position.

Indeed, there are calls, especially from donors and uncommitted consultants, for others to join the race.

These new candidates include: former first lady Michelle Obama, who evinces zero interest in running; former New York City Mayor Michael Bloomberg, who has said he might consider running if Biden falters; and presumably former senator, secretary of state and Democratic presidential nominee John Kerry.

Even one of 2016 Democratic presidential nominee Hillary Clinton’s top aides has publicly said that Clinton should think about entering the race.

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My own bet is that Clinton, in particular, is unlikely to run, but quite clearly is enjoying her role on Twitter as an ongoing Trump troll, giving her personal vindication.

Bottom line: the most important numbers to watch will be the polling emerging from the three early states. Those will indicate how the race is evolving.

The polling numbers – which span both before and after the most recent debate – show why one should look primarily at the results of the early states, rather than try to just use national numbers to consider how the nomination contest is shaping up.

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The state polls suggest that the Democratic race remains open. None of the top candidates appears to be consolidating support – but they’re not fading, either.

The opportunity for significant changes in the character of the race remain.

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