PHILADELPHIA — In a few short months, Donald Trump has shifted from a candidate who considered Speaker Paul D. Ryan a “very weak and ineffective leader” to a president who seems to be leaning on him to beef up his policy agenda.

A sign of that evolution came Thursday when Trump, speaking before congressional Republicans at their issues retreat here, embraced Ryan’s “border adjustment” proposal to tax U.S. imports instead of exports as a way to pay for his border wall.

“Paul Ryan and other leaders in Congress and I and Mike Pence … we’re working on a tax reform bill that will reduce our trade deficits, increase American exports and will generate revenue from Mexico that will pay for the wall if decide to go that route,” Trump said.

[Trump Seems to Embrace Key GOP Tax Proposal]

The president doubled down in a tweet Friday morning. “Mexico has taken advantage of the U.S. for long enough. Massive trade deficits & little help on the very weak border must change, NOW!” Trump wrote.