Democrats have an opportunity to reinforce the social safety net, address climate change and combat wealth inequality, without burdening future generations with new debt or stoking inflation. But as they hammer out their “reconciliation” package, they are in danger of falling tragically short.

President Biden promised to roll back President Donald Trump’s tax cuts. He pledged to fully pay for his Build Back Better agenda. Democrats are now in danger of violating both pledges.

To satisfy moderates, especially Sen. Kyrsten Sinema (D-Ariz.), Democrats are reportedly looking to fund their reconciliation bill without raising top-line tax rates on wealthy individuals and profitable corporations. They are instead considering imposing a corporate alternative minimum tax, creating a 15 percent minimum tax for multinationals’ overseas profits and curbing tax evasion by pumping up the IRS. Democrats reckon these sorts of measures would still cover the roughly $2 trillion over 10 years they believe their final package would cost.

Well, maybe. A corporate alternative minimum tax, international tax reform and enhanced IRS enforcement are good policies on their own merits, and they could certainly raise substantial amounts of revenue. But relying on more effective tax collection for $700 billion in new revenue — over a third of the package’s size — is a massive risk. Much would depend on whether Congress gave the IRS all the tools it needed, such as new bank reporting requirements, and on whether future Congresses maintained the agency’s funding. If Republicans took one or both chambers of Congress, they could easily undercut the IRS in government funding bills, as they have in the past, blowing a hole in the Democrats’ revenue projections. Even without GOP sabotage, the IRS would have to follow through on the technological upgrades, hiring and other measures necessary to improve its enforcement. Maybe that would translate into $700 billion. Maybe not.

More than the national debt is at stake. The Trump tax cuts were unconscionable giveaways to rich Americans at a time of growing wealth inequality and wide budget deficits. Raising top tax rates is a reliable way to boost government revenue — but also to make the economy fairer by redressing the Trump tax cuts’ wild distributional imbalances, especially if paired with a child tax credit for lower-income families. If holdout Democratic senators can’t stomach raising the corporate tax rate from 21 percent to 28 percent, as Mr. Biden favors, how about compromising on 25 percent — the number Republicans such as Sen. Mitt Romney (Utah) and former House speaker Paul Ryan (Wis.) once proposed?

With only 50 Democratic senators, Mr. Biden remarked Thursday, “every one is a president. Every single one. So you gotta work things out.” In fact, only Mr. Biden is the president, and he should insist at least on substantive tax reform and action on climate change. If Democrats did just those two things, it would be a major accomplishment. If they neglect either, it will be a rare opportunity squandered.