Divisions within the Democratic Party deepen as progressives gain more strength.

As Joe Biden pledges to unite a fractured nation, the incoming president faces deep divisions on the national front and widening fissures within the Democratic Party.

“His ability to maneuver between a rock and a hard place will be the test of his presidency,” Thomas Edsall, a veteran political journalist and professor at the Columbia School of Journalism, told ABC News.

In the wake of President Donald Trump‘s false claims that the election was stolen from him, a recent poll shows that roughly three-quarters (77%) of Trump backers say Biden’s win was due to fraud, despite there being no evidence to back this up.

These tensions over the election compounded two weeks ago when a pro-Trump mob, which the president helped incite, violently stormed the U.S. Capitol, temporarily disrupting the certification of Biden’s election win. The siege resulted in the death of five people.

“How is it possible to govern when such a large number of voters believe that the election was stolen?” Edsall said. “(Biden) says he wants to unify the country, but the opposition is really entrenched at this point in a way that I don’t think it’s ever been before,” he said.

Meanwhile, divisions within Biden’s own party continue to widen as the progressive wing of the Democratic Party gains more strength in Congress and a civil rights movement continues to activate a wide and powerful coalition of advocates and activists.

“There have already been pushes for us to be more moderate, to slow down to wait for change to happen,” Alicia Garza, co-founder of Black Lives Matter, said during a virtual inauguration eve event on Tuesday hosted by Busboys and Poets. “… [but] we don’t actually have that much time to make the changes that we want to see, and that is why we cannot rest on our laurels.”

Democrats retained control of the House and gained a slim majority in the Senate, with incoming Vice President Kamala Harris as the tie-breaking vote — a balance of power that is key to a Biden agenda. But that could all change after the next midterm election.

Dr. Cornel West, a professor of African American studies at Harvard University and a prominent supporter of the Bernie Sanders presidential campaign, told ABC News that he wants to give the incoming administration a chance, but he begins with “a note of suspicion.”

“We’ve got to raise our voices to make sure that the accountability is real. … we have to be associated with organizations that bring pressure to bear on not just Biden, not just Harris — on the whole, Democratic Party that is still very much shaped by its corporate wing,” West said.

The Democratic Party has always faced a “left-right divide,” but it has become “more acute” in recent years, Edsall said.

The Congressional Progressive Caucus has become more powerful with influential members, including the so-called “squad” — the group of progressive congresswomen of color, all of whom won their re-elections in 2020, despite relentless attacks from Republicans and Trump. The caucus also includes newly-elected progressive insurgents like Reps. Cori Bush and Jamaal Bowman.

And Sanders, the Vermont senator whose progressive presidential platform helped move the Biden campaign to the left, is set to become the chairman of the powerful Senate Budget Committee, where he will be “in a very strong position … to at least set initial priorities,” Edsall said.

Asked how the movement’s strategy could shift under a Biden White House, Garza said that under a more progressive administration, “what we are trying to do is set the menu. We are not just trying to sit at the table.”

Based on Biden’s nominations so far, the incoming Cabinet would be the most diverse in history and while that has garnered widespread praise, some progressives sounded the alarm over lobbying and corporate ties within the incoming administration.

“I just hope we don’t move from a neo-fascist version of greed and lies to a neo-liberal version of greed and lies,” West said.