Just over half of voters who say they intend to support former Vice President Joe Biden Joe BidenThe Memo: Warning signs flash for Trump on debates Senate Republicans signal openness to working with Biden National postal mail handlers union endorses Biden MORE in the presidential election say their minds are made up, according to a new poll, a potentially worrying sign for Biden as he works to gin up support ahead of a likely general election match-up with President Trump Donald John TrumpBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Military leaders asked about using heat ray on protesters outside White House: report Powell warns failure to reach COVID-19 deal could 'scar and damage' economy MORE.

A Grinnell College national poll released Wednesday morning showed that 55 percent of likely voters who said they would back Biden say their minds are made up, while 43 percent said they could be persuaded to support a different candidate.

Trump holds much stronger sway over his supporters in the poll, with 82 percent of likely voters who say they will cast their ballot for Trump saying their minds are set and only 17 percent saying they are still persuadable.

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Still, Biden fares better in the question than Sen. Bernie Sanders Bernie SandersSenate Republicans signal openness to working with Biden Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll Schumer, Sanders call for Senate panel to address election security MORE (I-Vt.), over whom he has developed a virtually insurmountable delegate lead in the Democratic primary. Fifty percent of Sanders supporters said their minds were made up, while 44 percent said they could be convinced to back someone else.

The former vice president holds a 47-43 percent lead over Trump in a hypothetical match-up, a 4-point advantage just beyond the poll's margin of error of 3.5 percentage points.

Nevertheless, the poll is bound to fuel further speculation over Democratic enthusiasm for Biden, a centrist who is still working to make inroads among progressive groups and voters heading toward the crucial summer months of campaigning.

Democratic handwringing first began after an ABC News-Washington Post poll released this week found that 74 percent of those supporting Biden are doing so enthusiastically, compared to 86 percent of Trump supporters.

Among those figures, 55 percent of registered voters backing Trump are "very" enthusiastic in their support, with 31 percent "somewhat" enthusiastic, while just 28 percent of registered voters in favor of Biden are "very" enthusiastic, compared to 46 percent who are "somewhat."

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“The enthusiasm gap recorded in the poll is troubling,” Brad Bannon, a Democratic strategist, said of the poll. “Everything the president has done since his inauguration was designed to fire up the Republican base, and the effort has paid off.”

Exacerbating fears further is the fact that Biden’s campaign has been largely sidelined by the coronavirus pandemic, relegating the former vice president to digital outreach while Trump addresses the nation in daily White House briefings.

However, Biden allies have expressed confidence that the campaign’s efforts to broaden its appeal will pay off in November.

“When this is all said and done, I think we’ll all be on the same page,” said one longtime ally who is in touch with the campaign.

The Grinnell College national poll, which was conducted by Selzer & Co., surveyed 777 likely voters from March 27 to 30 and has a margin of error of 3.5 percentage points.