The past two Democratic presidencies, and to some extent Mr. Trump’s as well, have followed this quick-burn model. After passage of the 2009 economic stimulus package with three Republican votes in the Senate, most of Barack Obama’s notable accomplishments fell into the 212-day window during which his party had a 60-vote Senate majority, and most of Bill Clinton’s into the two years in which his party had a congressional majority and was able to use the budget reconciliation process to bypass the filibuster. After midterm backlash, and even after the two presidents crawled back to win re-election, the remaining six years of each administration were lost to hand-wringing over the budget deficit and compromises on conservative priorities like welfare reform, with a few tactical achievements along the way. Only later in their presidencies did either master the other tools of governance, regulations and personnel.

Backlash, even more than the Senate filibuster or an unfriendly Supreme Court, has been the most consequential force in American politics in the era of high partisan polarization. As Eric Patashnik of Brown University, a leading scholar of why policies succeed or fail, wrote recently, “reducing the odds and potency of backlash is critical to the political sustainability of activist government.”

Avoiding backlash is an uninspiring goal, but if the next presidency follows the Clinton-Obama pattern, it would be a disaster for the policy aspirations expressed by all the Democratic candidates. If Democrats hold a Senate majority at all, it will be narrow, and conservative senators like Joe Manchin of West Virginia will set the boundaries of what’s possible. Major initiatives will need time for policy development, congressional deliberation, rollout and further refinement. The Affordable Care Act, for example, could have been far more effective if Congress had been able to revisit and fix it, just as it did with Social Security roughly every two years after its initial passage.

And there will be much more than legislation on the next president’s early to-do list. Trump-era regulations have slowed progress on climate change and reopened the student loan system to predatory for-profit colleges. Reversing that regulatory shift will take time for public comment and litigation, so a new administration will have to get started quickly. Career employees with expertise and experience have been driven out of many federal agencies; rebuilding that governing capacity and rooting out unqualified Trump appointees will be a major project. So will rebuilding the United States’ standing in the world and reversing the damage caused by four years of unchecked corruption.

If the next president achieves even a quarter of what each candidate has promised in the first two years in office, it will be a miracle. And a setback in the 2022 midterms would have long-lasting consequences. Republican-held Senate seats in Florida, Iowa, Missouri, North Carolina, Ohio, Pennsylvania and Wisconsin will be in play, with incumbents retiring or likely to retire in several. Losing those or other seats is likely to put the Senate out of reach for the rest of the decade. On the other hand, a Democratic president who could make gains in the midterm might have a real chance to build out a full progressive agenda over time.