Donald J. Trump once declared that campaigning for "substantial" changes to Medicare would be a political death wish.

But with Election Day behind them, emboldened House Republicans say they will move forward on a years-old effort to shift Medicare away from its open-ended commitment to pay for medical services and toward a fixed government contribution for each beneficiary.

The idea rarely came up during Mr. Trump's march toward the White House, but a battle over the future of Medicare could roil Washington during his first year in office, whether he wants it or not.

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