Six months after voters sent Republicans in large numbers to Congress and many statehouses, it is possible to see the full landscape of destruction that their policies would cause — much of which has already begun. If it was not clear before, it is obvious now that the party is fully engaged in a project to dismantle the foundations of the New Deal and the Great Society, and to liberate business and the rich from the inconveniences of oversight and taxes.

At first it seemed that only a few freshmen and noisy followers of the Tea Party would support the new extremism. But on Friday, nearly unanimous House Republicans showed just how far their mainstream has been dragged to the right. They approved on strict party lines the most regressive social legislation in many decades, embodied in a blueprint by the budget chairman, Paul Ryan. The vote, from which only four Republicans (and all Democrats) dissented, would have been unimaginable just eight years ago to a Republican Party that added a prescription drug benefit to Medicare.

Mr. Ryan called the vote “our generation’s defining moment,” and indeed, nothing could more clearly define the choice that will face voters next year.

His bill would end the guarantee provided by Medicare and Medicaid to the elderly and the poor, which has been provided by the federal government with society’s clear assent since 1965. The elderly, in particular, would be cut adrift by Mr. Ryan. People now under 55 would be required to pay at least $6,400 more for health care when they qualified for Medicare, according to the Congressional Budget Office. Fully two-thirds of his $4.3 trillion in budget cuts would come from low-income programs.