Here we are, just four weeks past Election Day, and renewed skirmishing has already broken out within the Democratic Party. Having mostly rallied behind Joe Biden after the primaries, progressives now want their policies and their people embedded in the new administration.

Consider this: One of the brightest stars in the progressive firmament, Representative Alexandria Ocasio-Cortez, recently led a rally outside the Democratic National Committee headquarters in Washington. Yes, the Democratic headquarters.

It’s almost as if the primaries had not ended.

The pressure is unrelenting and multidirectional. New Consensus, founded by Ms. Ocasio-Cortez’s former campaign manager, last month issued a seven-page manifesto filled with ill-considered and legally controversial ideas like ordering the Federal Reserve to make trillions of dollars of low-interest loans directly to businesses and projects.

And cognizant of the old mantra that “personnel is policy,” progressives are forcefully pushing their candidates for key positions while working to demonize other potential appointees not considered sufficiently woke and criticizing some of those already announced.

All of this lands Mr. Biden in a substantially more difficult position than what President Barack Obama faced after his victory in 2008.

Twelve years ago, the two principal combatants for the nomination — Mr. Obama and Hillary Clinton — sat nearly atop each other on the ideological spectrum. Fast forward to 2020 when the final contestants — Mr. Biden and Senator Bernie Sanders — disagreed over issues ranging from “Medicare for all” to the Green New Deal.

Mr. Biden, of course, emerged victorious, a clear demonstration that a majority of Democrats want a party that balances admirably ambitious goals with political reality and sound analytical footing.

In order for Mr. Biden to succeed, Democrats need to avoid friendly fire. All committed Democrats agree that Black lives matter, that every American is entitled to good health care and that climate change represents our greatest existential threat. And no, more far-left candidates would not have led to a better result in November’s contests for the House of Representatives and the Senate. Indeed, many have attributed Democratic candidates’ underperformance to the room Republicans were given to tie even centrist Democrats to meritless notions like defunding the police and socialism.

As Senator Jon Tester of Montana wrote in his recent memoir, “we are getting whupped in the messaging war.” That showed even in Minnesota, where Representative Ilhan Omar, a member of the heavily left-leaning squad in the House, appears to have run substantially behind Mr. Biden in her district.

Undaunted, progressives continue to warn Mr. Biden that he risks losing their support if he doesn’t push their agenda. But that ambition appears likely to collide with a Republican-controlled Senate.

Trying to push Mr. Biden into embracing policies that he rejected during the primary season risks any success in a Congress that is likely to be divided.

Recognizing that challenge, progressives are urging the incoming president to use his executive powers to achieve their agenda. I’m totally onboard with a muscular presidency, but forcing through policies that voters clearly rejected by executive order will jeopardize Democrats’ chances in elections to come.

In any event, the most important initiatives require legislation, and here again, progressives have already warned Mr. Biden against making toxic deals with a Republican-controlled Senate. Ironically, the harder the left pushes its agenda, the more intransigent it becomes and the more it risks achieving nothing.

Progressive history has not treated kindly President Bill Clinton’s decision to work with Republicans after their enormous midterm victory in 1994. But taxes got cut, the budget was balanced for the first time in decades, and the late 1990s is remembered as a period of strong prosperity.

It will be harder this time.

For those still mentally residing in those halcyon days when Mr. Clinton could achieve compromise, remember this: Mr. Obama’s signature legislative accomplishments — the Affordable Care Act and his stimulus package — received just three Republican votes (all of which were limited to Senate passage of the stimulus), despite countless hours of negotiations between the two sides.

So far, the president-elect has been deftly navigating this thicket. In the economic arena, all sides have heaped praise on Janet Yellen, his nominee for Treasury secretary, who combines stellar mainstream economic credentials with evident compassion for those left behind.

But as a mainstream economist, she has emphasized the importance of controlling the growth in the federal budget deficit and has also spoken up for free trade. Progressives need to understand that the Democratic Party is a big tent and not every nominee will embrace every morsel of leftist dogma.

Ms. Yellen faces daunting challenges, beginning with the push to win passage of another desperately needed stimulus bill. That bill should include aid to state and local governments, and more money for struggling businesses and unemployed Americans. She will need to journey to Capitol Hill with the authority to negotiate the best package possible.

If we Democrats want to avoid typical midterm election losses, if we want to retain the White House in 2024, and perhaps most important, if we want to at least attempt to address the nation’s many challenges, we need to check ideological purity at the door and focus on not letting perfect be the enemy of the good.

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