PHILADELPHIA (AP) — President Donald Trump called on fellow Republicans to help him enact "great and lasting change" during a party retreat Thursday but offered few details. Later his spokesman said the president will seek a 20 percent tax on Mexican imports to pay for a proposed border wall.

The president was greeted by cheers as he took the stage in a hotel ballroom, telling senators and House members, "This Congress is going to be the busiest Congress in decades — maybe ever."

He addressed lawmakers shortly after Mexican President Enrique Pena Nieto canceled a trip to Washington next week for his first meeting with the new president due to their disagreement over which of their countries would pay to build Trump's promised wall on the border between them.

The wall is part of Trump's plan to halt illegal immigration to the U.S., and he has long insisted that Mexico will pay. Pena Nieto insists his country will not.

On the flight back to Washington, White House spokesman Sean Spicer told reporters traveling with the president that Trump will seek to impose a 20 percent tax on Mexican imports to pay for the wall. Congressional approval would be needed for such a step.

In Trump's remarks to lawmakers, he cast the cancellation of his engagement with Pena Nieto as a mutual decision, saying they had "agreed to cancel our planned meeting." Trump had tweeted early Thursday that "it would be better to cancel the upcoming meeting" given Pena Nieto's unwillingness to pay for the border wall.

Trump's election put Republicans in control of both the White House and Congress for the first time in more than a decade. Yet Trump's often fluid ideology has sometimes put him at odds with his own party, making agreement on issues including a tax overhaul and entitlements no guarantee.

Addressing fellow Republicans, the president spoke about his agenda in broad terms and then skipped a planned question-and-answer session. He gave the lawmakers no specific marching orders for tackling the repeal and replace of "Obamacare," one of the most complicated issues Congress is expected to tackle this year.

Later Thursday, Trump planned to use his executive authority to continue implementing his agenda.

The White House is considering steps to commission a probe of widespread voter fraud, restrict the flow of refugees to the U.S., and negotiate individual trade deals with countries that signed the proposed Trans-Pacific Partnership trade pact. Trump took steps earlier this week to withdraw the U.S. from TPP, saying the agreement puts American workers at a disadvantage.

Trump's brief trip to Philadelphia marked his first flight on Air Force One, the familiar blue and white government plane that has long ferried presidents around the country and the world. Spokesman Sean Spicer described Trump — who traveled throughout the campaign and the transition on his own private jet — as being "in awe" of the presidential aircraft.

Trump saluted as he walked off his Marine helicopter and chatted with an Air Force officer who escorted him to the steps of the plane. He climbed the steps slowly but did not turn around and wave as presidents often do.