Managing a Divided Democratic Party is a Test for Joe Biden

Ever since President Trump won the White House in 2016, a shocked Democratic Party had been united behind the mission of defeating him. Four years later, with the election of Joseph R. Biden Jr., the divides that have long simmered among Democrats are now beginning to burst into the open, as the president-elect confronts deep generational and ideological differences among congressional lawmakers, activists and the party’s grass-roots base.

The fault lines began to emerge within hours of Mr. Biden’s victory. Moderates argued that his success, particularly in industrial Midwestern states that Mr. Trump seized from the Democrats in 2016, was proof that a candidate who resisted progressive litmus tests was best positioned to win back voters who had abandoned the Democratic Party. Those tests included single-payer health care, aggressive measures to combat climate change and expanding the Supreme Court.

“The progressives said we need a base candidate,” said Rahm Emanuel, the former mayor of Chicago and White House chief of staff under President Barack Obama, referring to a nominee who appeals to the left wing of the party. “No we didn’t. We needed someone to get swing voters. If you campaign appropriately, you can make that a governing transformation.”

Moderate Democrats said they were hopeful the urgency of the problems confronting the nation would delay the inevitable reckoning the party faces between its ideological wings. Beyond that, they said that a disappointing showing by Democrats in congressional races — the party lost seats in the House and faces a struggle for even narrow control of the Senate — would give liberal Democrats less of a platform to push Mr. Biden to the left.

After a fiery call among members of the House Democratic caucus, in which some argued that progressives who have entertained ideas like defunding the police or “Medicare for all” had cost the party congressional seats, some Democratic leaders pushed further away from the left wing.

Representative Conor Lamb, a moderate from Pennsylvania who survived a difficult Republican challenge, said the results should be a wake-up call to the left.

“What we heard from a lot of our constituents was that they do not like the Democratic message when it comes to police in Western Pennsylvania, and when it comes to jobs and energy,” he said. “And that we need to do a lot of work to fix that.”

But after four years of pent-up frustration and energy, that may prove unlikely. By every early indication, Mr. Biden’s election has emboldened progressive energy, no matter the setbacks in the congressional races. There is an up-and-coming generation of elected Democratic officials who have been waiting in the wings, eager to take the lead in formulating a platform for the party.

After supporting Mr. Biden as a means of defeating Mr. Trump, younger and more progressive Democrats who have gained a foothold in Congress and among party activists are skeptical about his future administration. Senator Elizabeth Warren of Massachusetts, setting policy terms in a statement after Mr. Biden was declared victorious, said: “A Band-Aid approach won’t get the job done. We…