A good showing for Democratic long-shots Dean Phillips, a Minnesota congressman, and self-help author Marianne Williamson would likely fuel concerns that Biden is weak heading into the general election battle.
Biden supported a move by national Democrats to move their first primary election to South Carolina, a more diverse state than New Hampshire.
But his New Hampshire supporters can still vote for him because many U.S. states allow voters to write the name on a ballot paper of a candidate who is not officially running. That candidate wins if they receive more votes than the registered candidates.
Biden is expected to win the primary with plenty of write-in votes, but whether his victory will be seen as sufficiently strong will probably be debated by his rivals.
A CNN/University of New Hampshire poll, opens new tab released on Sunday showed Biden drawing 63% of the primary vote in that state, with Phillips at 10%.
Democratic supporters have sought to temper expectations. They point to the 39% support that Senator Lisa Murkowski won during a write-in election in 2010 as a sign of what Biden might achieve.
"A win is a win," said U.S. Rep. Ro Khanna, a Biden supporter who attended a number of roadside events promoting the write-in option.
Phillips, running an insurgent campaign against Biden based on the notion that he is weak enough to be vulnerable to defeat by likely Republican nominee Donald Trump, said the incumbent would need a much stronger performance to erase doubts.
At a Bloomberg News roundtable, Phillips said Democratic presidents Bill Clinton and Barack Obama got 80% of the vote or more in their re-election bids in 1996 and 2012.
Those two elections, however, were when their names were actually on the ballot. "We are proceeding with a dangerous coronation," Phillips said.
Phillips has made some inroads with New Hampshire voters but whether it will be enough to give him a strong boost of support on Tuesday was unclear.
Margaret Flynn of Nashua, New Hampshire, said she voted for Biden in 2020 but expects to vote for Phillips.
"I think Biden's too old to be president, I think he's kind of outdated, and I'm not sure he can beat Trump," she said after a Phillips event in Nashua.
The White House has repeatedly dismissed concerns about 81-year-old Biden's age, though polls show voters are concerned about it.
Benjamin Bolger of Hanover, New Hampshire, said the change to South Carolina has upset a great tradition in New Hampshire where candidates meet the voters at diners, town hall events and the like.
"I think no one should skip the New Hampshire primary and I'm disappointed," he said after attending a Williamson event in Manchester. "I'm disappointed that President Biden is not here."
(Reporting by Steve Holland; Editing by Heather Timmons and Alistair Bell)
By Steve Holland
