MANCHESTER, N.H. — The two top finishers in last week's Iowa caucuses, 40 years apart in age and representing opposite ends of the Democratic Party's ideological spectrum, are heading for a showdown in Tuesday's primary — each taking increasingly aggressive swipes at the other.

But there is an unusual twist to the new rivalry between Bernie Sanders and Pete Buttigieg.

The senator from Vermont and former mayor of South Bend, Indiana, are not fighting to win over the same pool of New Hampshire voters, though many likely to vote here remain undecided. Rather, according to strategists in both campaigns, each is trying to energize his supporters by vowing to block the other from winning the Democratic nomination — with Sanders, the democratic socialist, portraying Buttigieg as a captive to his billionaire donors, and the more centrist Buttigieg railing against Sanders as a "my way or the highway" leftist.

That effort burst into full view Sunday, when the candidates hopscotched across the snow-covered state.

At a canvass launch in Plymouth, Sanders accused Buttigieg of being in the pocket of his wealthy donors. "Our views are different," Sanders said, repeating an argument he has made frequently in recent days.

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"Pete has raised campaign contributions from over 40 billionaires," he said, adding that those donors are drawn to Buttigieg precisely because he will not, in Sanders's view, take on "the corporate elite."

Buttigieg, meanwhile, continued to paint Sanders as divisive.

"I respect Senator Sanders, but when I hear this message go out that you're either for revolution or you've got to be for the status quo, that's a vision of the country that doesn't have room for most of us," Buttigieg said.

At another event Sunday, he pushed his usual critique further, calling Medicare-for-all irresponsible. "As long as we're willing to have some common sense here, we can deliver the biggest change to American health care in a half-century," Buttigieg said But what we could do without is a plan so expensive that Senator Sanders himself freely admits he has no idea how it's supposed to be paid for."

The growing friction between Sanders and Buttigieg comes as both are taking fire from other rivals, who are scrambling to finish in the top three in New Hampshire.

Former vice president Joe Biden, who finished far behind in Iowa and has slipped in New Hampshire polls, has been attacking Sanders as too liberal and Buttigieg as too untested to beat President Donald Trump. Sen. Amy Klobuchar, D-Minn., likewise has questioned both men on similar fronts.

But strategists say Tuesday's election could largely come down to how Sanders and Buttigieg play off each other.

"Sanders is kind of solidifying his lead among the progressive wing of the Democratic Party," said Andrew Smith, director of the University of New Hampshire Survey Center. And Buttigieg, coming from the opposite direction, has become the "second choice for everybody," Smith added. "He's the acceptable candidate across the board."

The rivalry has been months in the making, in part due to the extreme contrasts between the two.

Sanders, 78, would be the oldest U.S. president in history; Buttigieg, 38, would be the youngest. Sanders would be the most liberal nominee in decades; Buttigieg would mark a return to a pragmatic approach more typical of recent nominees. Sanders has galvanized young people and the working class, including nonwhite voters, but has struggled to win over people over 50; Buttigieg appeals to older and more affluent white voters, as well as some in rural areas; but he has struggled badly with nonwhite voters.

Former South Bend, Ind., Mayor Pete Buttigieg in Keene, N.H. Bonnie Jo Mount / The Washington Post
Former South Bend, Ind., Mayor Pete Buttigieg in Keene, N.H. Bonnie Jo Mount / The Washington Post

"I don't hear people that are saying they've winnowed it down to those two," said Jim Demers, a longtime New Hampshire Democratic strategist supporting Biden.

The outcome in New Hampshire could have a significant effect on the presidential contest moving forward, potentially breaking their effective tie in Iowa and shaping the way voters view the candidates in the next two nominating contests: Nevada and South Carolina.

That is reflected in the disdain their supporters feel for one another.

At a Saturday night dinner, Buttigieg criticized Sanders, saying, "When the president is this divisive, we cannot risk dividing Americans further, saying that you must either be for a revolution or you must be for the status quo."

"Let's make room for everybody in this movement," he added.

Sanders's followers in the audience booed and chanted: "Wall Street Pete!" When Sanders spoke later, he told the crowd he was excited to have won the most votes in the Iowa caucuses, prompting a roar from his supporters.

"Our campaign is off to a great start. We are pleased, we are excited that we won in Iowa the popular vote, by 6,000 votes," declared Sanders, referring to one of several tallies in the caucuses, which Iowa Democratic leaders were still examining. His words didn't sit well in the Buttigieg section. Rows of Buttigieg supporters rose as Sanders declared himself the victor. "Boot-edge-edge!" they chanted, enthusiastically hoisting "Pete" signs.

Sanders decisively defeated Hillary Clinton here four years ago, and polls show he has remained popular, even if his level of support has slumped far below what it was when he was the sole anti-establishment option. Buttigieg has tried to counter Sanders' familiarity with post-Iowa momentum, drawing large crowds and marquee endorsements.

The Iowa results have added to the grating nature of the competition. Both candidates have claimed victory (a winner has not been officially declared, but Buttigieg held a narrow delegate lead even as Sanders won the popular vote). In the aftermath, some prominent Sanders supporters have, without presenting clear evidence, made speculative claims tying problems with the vote count to Buttigieg.

Longtime Democratic operatives in New Hampshire said that what happened in Iowa has become muddled in the minds of many local Democrats. And the state's contrarian tendencies, coupled with ability of independent voters to cast ballots in the primary, adds even more uncertainty to the ordering here.

But it's clear the Buttigieg and Sanders campaigns view each other as targets more than they did before.

At Friday night's debate, Buttigieg railed against "a politics that says, if you don't go all the way to the edge, it doesn't count, a politics that says it's my way or the highway."

"Are you talking about Senator Sanders?" a moderator asked him.

"Yes," replied Buttigieg. "Because we've got to bring as many people as we can into this process."

Sanders presented a competing view of how to foster unity and defeat Trump.

"The way you bring people together is by presenting an agenda that works for the working people of this country, not for the billionaire class," continuing a line of attack he started in the morning when he read news headlines about Buttigieg at a breakfast event.

Some Sanders campaign officials were thrilled with the newly aggressive posture. His aides have been seeking to amplify his arguments. And some of them feel there is another benefit to confronting Buttigieg — his voters might flee to Biden or Klobuchar, splitting up support in the party's centrist wing.

"He certainly has consolidated the corporatist lane at this point," said Sanders senior adviser Jeff Weaver, speaking of Buttigieg. "If there's an exchange between a progressive vision and a corporatist vision, the progressive vision is going to win out."

Buttigieg's aides have also been aggressive, if not always as direct. "We have exactly one shot to beat Donald Trump. We're not going to do it by overreaching. We're not going to do it by division," Buttigieg's deputy campaign manager Hari Sevugan said on a call with supporters Wednesday night. "We need Pete to be the nominee."

Even before this week, tensions between Buttigieg and Sanders were rising. Many of Buttigieg's harshest critics — the protesters who interrupt his events to critique his handling of race and climate — are Sanders supporters. Some members of the South Bend Chapter of Black Lives Matter, who followed Buttigieg around the country to protest racial inequity in his city, are also leaders in the local chapter of Our Revolution, a pro-Sanders group.

The audiences the two candidates tend to draw on the campaign trail are different. Sanders tends to attract youthful, raucous crowds. They roar his name, rail against the establishment, and cheer on high-profile surrogates doing the same. Often, they share stories of economic suffering.

Attendees at Buttigieg events are older and largely white. They applaud politely. They cheer him when he suggests his movement is open to everyone, particularly when he argues for reaching out to dispirited Republicans.

Some Sanders advisers earlier had privately welcomed the rise of Buttigieg, reasoning that there was much less overlap between their sets of voters than there was between supporters of Buttigieg and two other Sanders rivals — Biden and Sen. Elizabeth Warren, D-Mass.

They figured Buttigieg's ascent would not come at Sanders' expense, which some of them feared would be the case with a Biden or Warren surge. But now, Sanders and his team are increasingly focused on trying to finish ahead of Buttigieg to blunt his gains and avoid being overshadowed by a fresher option.

Buttigieg, meanwhile, has made Sanders a focal point of his fundraising emails to supporters. "Can you make a donation today to help us take on Bernie in New Hampshire? He's got one of the most well-funded organizations in politics and has been building his email list for years," said one his campaign sent this past week.

There is one unexpected layer to the Sanders-Buttigieg story.

Long before they were presidential rivals, Buttigieg won an essay contest in 2000 as a teenager with a piece critical of compromising centrist politicians — and praising Sanders for his commitment to unifying.

Some Sanders supporters have used the essay as ammunition against Buttigieg on social media. It came up again last week when a questioner at a CNN town hall asked Buttigieg whether he had become an example of what he had once condemned.

"What I really admired about Senator Sanders — and still do — is his consistency and willingness to say exactly what he believes," Buttigieg said. "It doesn't mean I agree with him. I didn't agree with him on everything then and don't agree with him on everything now. ... And I think everybody, left, right, and center, ought to come into the public square making the case for what we think is right."

This article was written by Sean Sullivan and Chelsea Janes, reporters for The Washington Post.