WE’RE FAMILIAR by now with the incentives that push against compromise. Members of Congress increasingly come from lopsided districts. That’s due both to gerrymandering and population sorting. The effect is to elect true believers who see no wisdom in the opposing argument — and to discipline those who might compromise with the threat of doctrinaire primary opponents.

Meanwhile, interest groups from the National Rifle Association to the Sierra Club value purity over governing. The media are increasingly fractured, so that citizens can choose to come into contact only with views they will find congenial. The political parties gain ground by vilifying and caricaturing each other.

Most of all, what’s required of politicians as the baby boomers age is difficult. We use a shorthand: Republicans want entitlement reform; Democrats want higher taxes. But, really, neither of them wants either of those. Polls show that voters like deficit reduction in theory but don’t favor the policies needed to bring it about: tax hikes and a scaling-back of promised retirement and health benefits.

President Obama’s proposed budget has taken a big step toward acknowledging reality. No, it’s not big enough. As we noted on this page Thursday, it would allow retirement and health programs to grow so quickly that everything else — the “discretionary” part of the budget — would decline over the next 10 years from 8.3 percent to 4.9 percent of the economy. That’s highly unrealistic and, as it applies to national security, dangerous.

But Mr. Obama has injected a courageous note of realism where the Republicans so far have shown none. He has proposed modest revenue increases, and by a reasonable method: capping the value of tax deductions in a way that does not diminish the incentives for charitable giving or homeownership. He has proposed modest brakes on the growth of Social Security and Medicare, to the horror of more rigid members of his party.

Some Republican leaders responded dismissively. The entitlement reform was too “modest” to justify the tax hikes, Senate Minority Leader Mitch McConnell (Ky.) said. House Budget Chairman Paul Ryan (Wis.) said the Social Security reduction was simply “clarifying a statistic which does happen to save money.” The House Republican campaign honcho, Rep. Greg Walden (Ore.), had the gall to accuse Mr. Obama of “trying to balance this budget on the backs of seniors” — this, after Republicans have spent months attacking the president for not reining in entitlements.

The pooh-poohing would be easier to take if the GOP had a real-world plan of its own. Instead, it pretends it can balance the budget without raising taxes — but also without ever specifying the details of the spending that would be decimated, discretionary or otherwise. Mr. Ryan and others so far have wanted credit for fiscal prudence without political cost.

Plenty of Republicans know better, and it’s time for them to step up. There are adults in the party who understand that revenue will have to rise, entitlements will have to be reduced and, for anything to be accomplished, the political pain of both will have to be shared.

It was encouraging to hear Speaker John Boehner (R-Ohio) repudiate Mr. Walden’s irresponsible statement Thursday and say that he was “encouraged” by aspects of Mr. Obama’s budget. We’ve heard similar sentiments from other Republicans, speaking quietly and off the record. Now that Mr. Obama has anted up, it’s time for them to be a bit louder.