"They've got some problems right around the corner," said Jim Manley, once a top aide to former Democratic Senate Majority Leader Harry Reid.
Biden has made many of the changes he has clear authority to do by executive action. Landmines going forward include pushing laws on which the Democratic Party is divided, such as college debt relief, tax hikes and curbs on the energy industry.
Then there are the intractable policy fights that have defined American politics for a generation, including who can become a citizen, how easy it should be to vote, whether the government should pay for healthcare, and who should carry a gun.
Meanwhile, many tricky issues, from trade tariffs to China policy to tech oversight, are still under review at the White House.
Democrats are working to pass their economic stimulus package with or without Republican support before a critical mid-March deadline when expanded unemployment insurance expires.
The bill only needs a majority vote, because it will be passed as part of a process called reconciliation, but that requires every Democrat to side with the White House. Doubts are growing that the bill will include a provision raising the federal minimum wage to $15, which would sorely disappoint liberal Democrats.
"I've been shocked at how disciplined the Left has been; I'm not sure how much that's going to last," Manley said. "I can see there's some fissures developing."
Those cracks were on display when some Democrats, including Representative Alexandria Ocasio-Cortez of New York and Senator Elizabeth Warren of Massachusetts, criticized Biden after he said told a Feb. 16 CNN town hall he disagrees with members of his party who want to forgive $50,000 in student debt.
A comprehensive White House-backed immigration bill unveiled on Feb. 18 is not expected to pass the Senate; the second-ranked Democrat, Dick Durbin, is among those suggesting a less-ambitious effort that focuses on immigrants brought to the United States as children.
Republicans are reshuffling after the Trump years, said Paul Shumaker, a Republican strategist behind Senator Thom Tillis' hard-fought re-election in North Carolina.
Biden could unite them by overreaching on taxes and spending, he noted, while doing too little on these issues will disappoint some of his Democratic base.
"He's enjoying a honeymoon period, but everyone knows that honeymoon's going to come to an end," Shumaker said.
ELUSIVE REPUBLICAN SUPPORT
White House aides say the policy agenda they plan to push in the coming months has bipartisan voter appeal, and they believe Republicans in Congress could ultimately be forced to support it by their constituents.
"Is he going to be focused on winning every last Republican over No, of course not," said White House communications director Kate Bedingfield, a longtime Biden confidant.
"But is he going to reach out and speak to people on both sides of the aisle – is he going to work to put forward plans that meets the needs of people of both parties – yes, he absolutely is."
Biden's early polling numbers suggest that will be a challenge. Some 56% of Americans approve of his performance as president, according to a Reuters/Ipsos poll conducted in mid-February, but just 20% of Republicans.
The White House's bipartisan hopes lie in an infrastructure plan, still in the embryonic stages of development, that is expected to exceed the scale, scope and price tag of the roughly $1.9 trillion stimulus bill.
The measure is almost certainly going to both expand the deficit and require some tax increases, measures expected to spur opposition. It is likely to be peppered with measures on climate change, and could also include Biden's proposed subsidies for college, according to several people briefed on early conversations.
Putting the pieces together will be tough without a full senior staff, including Biden's pick for budget director, Neera Tanden, whose confirmation has run into Democratic opposition from Senator Joe Manchin, who also opposed including the minimum wage in the stimulus bill.
Nonetheless, the Left's expectations for Biden remain high.
"The administration came out bold and strong," said Luis Hernandez, a youth gun violence prevention activist who met with senior administration officials last week. "There's much more to be done."
(Reporting by Trevor Hunnicutt; Editing by Heather Timmons and Daniel Wallis) ((email@example.com; +1 646 223 7914; twitter.com/trhunnicutt; Reuters Messaging: firstname.lastname@example.org))