MANCHESTER, New Hampshire (CNN) Tuesday's primary in this Northeastern state may mark the final day of nearly 50 years of unparalleled influence for Iowa and New Hampshire as the one-two kickoff contests in the Democratic presidential nominating process.

In this year's presidential campaign, the distorting effects of providing such power to two virtually all-white states in an increasingly diversifying party have grown impossible to ignore. The vote-counting meltdown in Iowa's antiquated and haphazard caucus system -- a process used partly to circumvent New Hampshire's law requiring it to hold the nation's first primary -- has further underlined the flaws in the existing order.

For both of these reasons, the states seem less likely this year than in any recent contest to play their historic role of definitively winnowing the field of contenders.

Only twice since the Iowa-New Hampshire duopoly hardened into place in 1972 have Democrats chosen a nominee who did not win either. But the states' verdicts this year may face the greatest risk in decades of being second-guessed by the larger and more racially diverse states that follow.

"The nominee of the party will be someone who has convinced the voters of the Democratic Party, including voters of color, that they are the best person to beat Donald Trump," says Lily Adams, the communications director for California's Sen. Kamala Harris during her Democratic presidential bid. "If you have not done that second part, there is just no way to win. And, frankly, there shouldn't be."

Both near- and long-term questions are swirling around the privileged positions of Iowa and New Hampshire. The long-term question is whether the two states will continue to lead off the Democratic nomination process; Sunday on "State of the Union," Democratic National Committee Chairman Tom Perez told CNN's Jake Tapper that the party "absolutely" will revisit the question after November. The more immediate issue is whether the states will shape the remainder of the 2020 race as powerfully as they have most other recent Democratic contests.

In every contested Democratic presidential race since 1992, just two candidates emerged from Iowa and New Hampshire with genuine shots at the nomination. But this year, "I could see a scenario where four candidates go on to Nevada with at least some plausible path to the nomination," says Dante Scala, a political scientist at the University of New Hampshire who's the author of "Stormy Weather," a 2003 history of the New Hampshire primary.

The 2 states' rising influence

It's difficult to overstate how large a shadow Iowa and New Hampshire have cast over the nomination process since 1972. That was the year in which Democrats charted the modern calendar of an Iowa caucus followed by the New Hampshire primary. That sequencing reflected a détente between the two states, and the national party, over New Hampshire's traditional "first in the nation" primary status. New Hampshire state law requires it to hold its primary at least seven days "immediately preceding the date on which any other state shall hold a similar election."

Iowa structuring its election as a caucus, rather than a primary, was the distinction that allowed it to go first without objection from New Hampshire, notes Elaine Kamarck, a longtime member of the Democratic National Committee's rules committee and senior fellow at the Brookings Institution. "A long time ago the deal was struck that you could have a first caucus and a first primary," she says.

Since Iowa and New Hampshire established themselves as the first states on the calendar, only two Democrats have won the nomination without succeeding in either of them. George McGovern took the prize in 1972 despite finishing second to front-runner Edmund Muskie in both. In 1992, Bill Clinton won the nomination after losing both, but in a year when the candidates had written off Iowa to a home state hopeful, Sen. Tom Harkin, and New Hampshire's impact was clouded by local considerations as well: Sen. Paul Tsongas, from neighboring Massachusetts, won the primary here that year.

If anything, the combined influence of Iowa and New Hampshire has appeared to swell further in this century as media coverage of the initial results has intensified. As I've written, in the four contested Democratic presidential races since 2000, candidates who did not first win either Iowa or New Hampshire have won only five states in all, out of 200 opportunities. The Iowa winner, in fact, captured the nomination in each of those four races, a reflection of how much benefit candidates now accrue from establishing themselves as winners from the outset.

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By contrast, because evangelical Christians are more influential in Iowa than elsewhere, the winner of the Iowa Republican caucuses has ultimately lost the nomination the last three times. New Hampshire has been a better predictor for Republicans: The winner there has won the past three Republican nominations.

The two states' outsized influence this year drew unprecedented complaints from Democratic activists -- and even some presidential contenders such as former Housing and Urban Development Secretary Julian Castro -- frustrated that they do not represent the modern Democratic Party. Democrats today are increasingly centered in major metropolitan areas, yet Des Moines, the largest metro area in Iowa, ranks only as the 87th largest in the US, and Manchester-Nashua, the largest urban center in New Hampshire, places only 129th.

That disparity stood out even more glaringly after all the major candidates of color -- Castro, as well as African American Sens. Cory Booker of New Jersey and Kamala Harris of California -- were forced from the race before anyone voted. The primacy of Iowa and New Hampshire were not the only, or perhaps even the major, reason they were sidelined: Many thought their biggest problem was the fear among Democratic voters, including minority voters, that a nominee of color could not beat President Trump. But many around those campaigns felt that their challenge was compounded by the need to establish credibility in two predominantly white states before any diverse states weighed in.

That dynamic plays out in other ways too

The demise of those candidates illuminated the limits of the solution that Democrats have relied on since 2008 to balance Iowa and New Hampshire: moving Nevada, with its large Latino population, and South Carolina, where African Americans cast about three-fifths of the Democratic primary votes, to the third and fourth positions on the calendar. Both of those states -- especially South Carolina -- have mattered before and they are likely to exert a big influence again this year. But they will be choosing only among the candidates who still appear viable after the first two, predominantly white, states render their verdicts.

That dynamic not only weakened the candidates of color, but it's also threatening the white candidate most dependent on minority voters: former Vice President Joe Biden. Polling both nationally and in South Carolina has shown Biden holding a big lead among African American voters, especially older ones. His fate now rides on whether he can maintain that black support after his disappointing fourth-place showing in Iowa and the likelihood that he will again finish well behind Sen. Bernie Sanders of Vermont and former South Bend, Indiana, Mayor Buttigieg, the leaders in the latest CNN/University of New Hampshire poll here.

Sanders and Buttigieg face the opposite question: Can they maintain the success they've enjoyed in these two mostly white states once the calendar turns toward contests with larger minority populations, starting with Nevada and South Carolina through late February but rapidly extending through early March into highly diverse large states including Florida, North Carolina, Texas, Arizona and California, as well as Midwestern battlegrounds such as Michigan and Illinois?

Adams says the looming presence of those diverse states may make the results in Nevada on February 22 a better predictor of how March unfolds for the Democrats than the outcome Tuesday night in New Hampshire or last week's showing in Iowa.

"It's just going to be tough in a state [New Hampshire] that demonstrably does not represent the coalition to really start making an argument that you can pull together the coalition that you need to win the primary and, first of all, to do well in March," she said. "I think Nevada will be far more of an indicator than New Hampshire will be."

Fragmented coalitions

Sanders' standing with black voters encapsulates the larger uncertainty looming over his early ascent. So far there's little evidence he is expanding much, if at all, beyond the coalition he mobilized in 2016, which ultimately proved too small for him to win. At an event for the senator in Rochester on Saturday, almost everyone I spoke with in the crowd had supported him in 2016 as well. These voters expressed passionate admiration for his consistency and fervor.

"He's consistent, he's real, he's generous and he will never waver," Dean Kandilakis, an X-ray technician from Rochester, told me. "If he gets into office we can count on him to do what he says he's going to do."

But in New Hampshire, as in Iowa, it is rare to meet anyone at an event for the other candidates who is even considering Sanders. Almost invariably, those voters view him as too extreme and unelectable.

"I feel it's too much to use the words 'I'm a socialist' and ever win with people in this country," Alyson Steg, a teacher from Cambridge, Massachusetts, who had traveled to a Buttigieg event in Nashua, told me.

Of course, at events for candidates other than Biden it is equally common to hear voters describe the former vice president as too old and unsteady to beat Trump. (In the latest CNN/UNH poll, the share of New Hampshire Democratic voters who said Biden was the most likely to beat Trump had dropped to 22%, half its level from last July.) "Wonderful guy, but I think he's not the right person for the moment," Barbara Smith, a freelance corporate communications writer from Nashua, told me at the Buttigieg rally. "I think he doesn't have what it takes right now on every level. This is a young person's game."

In turn, voters considering the other candidates worry whether voters will elect a woman such as Klobuchar or Warren, or a candidate as young and with as little experience as Buttigieg.

"If I was undergoing surgery at Mass General, I'd want the guy who had done the surgery before," said John Broderick, a longtime Biden supporter and former New Hampshire State Supreme Court chief justice, who had traveled from North Andover, Massachusetts, to see him on Sunday.

After Tuesday, these arguments will roll into states that offer much more diverse demographic profiles than Iowa or New Hampshire. But the candidates will arrive in those later states with their positions very much stamped by the reactions of the preponderantly white electorates in the first two contests.

This could be the last time that happens. While New Hampshire's law requiring it to hold the first primary probably makes it impossible to dislodge, Iowa may be caught in a Catch-22: If the state switches to a primary, New Hampshire will adjust its date to go first, and if Iowa continues to hold caucuses, the Democratic National Committee likely won't let it start the nominating process.

"We probably can't outlaw caucuses altogether because there are states that will not adopt a primary for the presidential race," says Kamarck. "But I don't think we can anymore allow a caucus to be the first."

Kamarck predicts the most likely outcome for 2024 is that the New Hampshire primary will become the first Democratic contest, followed immediately by a primary in a more diverse state, probably South Carolina. That shift can't come too soon for the critics who say that the current calendar has unfairly suppressed the voices of the party's growing nonwhite voter base.

In the meantime, no question may loom larger from Tuesday's New Hampshire results than whether Nevada and especially South Carolina will come too late to provide Biden, the national Democratic front-runner through all of 2019, a final chance to revive his struggling campaign.