Since many potentially vulnerable Republican congressional seats are in safely blue states like California and New York, a serious effort to end Republican majorities would of course require expenditures that will have no impact on the Electoral College vote. In the standard way of thinking about trade-offs like these, it would be rational to tolerate a slight increase in the risk of losing the electoral vote if doing so would sufficiently increase the odds of ending congressional gridlock.

But because a campaign’s budget is not a fixed sum, the trade-off may be more apparent than real. As economists have long stressed, the amount that people are willing to pay for something depends on what they expect to get in return. Democratic donors understand that their biggest concerns can’t be addressed until Republicans lose their congressional majorities. They also understand that if the House doesn’t flip this year, there will be virtually no chance of it flipping in the 2018 midterm elections. And until Democrats win enough seats in state legislatures to undo Republican gerrymandering — which could take decades — a wave election is the only near-term hope.

The candidacy of Donald Trump offers a unique opportunity. If Mrs. Clinton made the case clearly in these terms, many donors would step up. Democrats could compete for every vulnerable Republican seat without diverting a single dollar from the Electoral College battle.

Some argue that money in politics doesn’t matter. That’s true in the sense that when both sides spend equally, their efforts tend to be mutually offsetting. But that’s why the current opportunity is unique. Democratic donors, who have already been giving generously, have both the means and the inclination to pay for an advertising blitz that Republicans probably cannot match this time around.

If Mrs. Clinton wins the presidency, she has pledged to appoint Supreme Court justices sympathetic to laws curtailing campaign spending. But this election is governed by current laws. A certain rough justice would be served if those laws helped dislodge the Republicans who favor them.

Again, the most urgent reason for a serious effort to flip the House is that longstanding Republican hostility to climate science has blocked steps that could parry the biggest threat to our planet’s survival. Estimates suggest that taxing carbon could slow greenhouse gas emissions by enough to stabilize global temperatures by the middle of this century. In a rational world, we would have long since taken that step. But Republicans have persistently refused even to discuss that possibility.

REPUBLICAN opposition to greater investment in clean energy and infrastructure refurbishment is rooted largely in their hostility to higher taxes. But supporting such investments would be less difficult than most people realize. That’s because of the seemingly plausible, but essentially false, belief that higher taxes would make it harder for prosperous people to buy what they want.