But if moderate Democrats are disposed to cooperate with the deal-maker in chief, they ought to exact a high price. On tax reform, for example, they should insist that Mr. Trump deliver tax relief to the middle class, not the wealthy, and that he jettison Mr. Ryan’s proposed border-adjustment tax, which would hit consumers and business with big price hikes. The administration needs to find better ways to pay for a sharp reduction in the corporate tax rate. Democrats don’t have to love big business to recognize that our antiquated tax system forces companies to pay much higher taxes than their overseas competitors. That makes American workers less competitive and gives our companies incentives to move investment abroad — and keep profits there — to avoid the higher rate.

Some die-hard Republican supply-siders would gladly abandon Mr. Ryan and simply add the cost of Mr. Trump’s enormous tax cut to the federal deficit. Democrats ought to hold their feet to the fiscal fire. One way is to close loopholes that cost hundreds of billions in lost revenue and, most economists believe, distort investment decisions. Another is to cap the value of tax deductions (as President Obama proposed) so that high-income people would receive no more tax relief than anyone else. More ambitiously, Democrats could go where Mr. Ryan apparently feared to go by proposing a consumption tax that has actually been tried and proved effective around the world — the value-added tax, with adjustments to ensure progressivity.

Democrats should also insist that Mr. Trump put new revenue on the table, specifically an economywide carbon tax. Otherwise, it will be difficult if not impossible to finance both a comprehensive tax overhaul and the nation-building infrastructure push Mr. Trump has promised. According to the Harvard economist Joe Aldy, a $25-per-ton carbon tax going up 5 percent a year could raise from $130 to $200 billion a year by 2030. Crucially for Democrats, it would also provide a powerful, market-based alternative for the Clean Power Plan and other regulatory policies for lowering carbon emissions that Mr. Trump and the Republicans want to scuttle.

Finally, pro-growth Democrats should be prepared to work with the White House to pass a major boost in spending on roads, air and seaports, railways, water systems, a “smart” electrical grid and other public goods that can support more robust economic innovation and investment. In return, they should demand that it’s actually funded, and that a significant share target people and communities left behind by changing technology and globalization.

Would Mr. Trump accept Democrats’ help on these terms? If he really wants to start racking up “wins” for his voters, he would. He’d have to share credit — a novel experience — with Democrats, who’d get points from swing voters for being pragmatic and competent. And they wouldn’t be constrained from fiercely opposing Mr. Trump on just about everything else.

If Democrats have a chance to help average working families and show they’re not obstructionists, they should take it. America doesn’t need two parties of no.