Donald Trump said on Sunday he is open to raising taxes on wealthy Americans, backing off his prior proposal to reduce taxes on all Americans.

"I am willing to pay more, and you know what, the wealthy are willing to pay more," Trump told ABC's "This Week."

Trump, who effectively sealed the Republican presidential nomination last week after the departure from the race of his two remaining rivals, has pledged to unify the party behind him.

But prominent Republicans remain deeply divided over the New York billionaire's candidacy. Republicans such as Paul Ryan, the top Republican in the House of Representatives, have distanced themselves from Trump over his proposal to temporarily ban Muslims from entering the United States.

Trump's fiercely anti-trade rhetoric is also at odds with the views of Ryan and many other pro-business Republicans.

Underscoring the party divisions, former Republican vice presidential candidate Sarah Palin, a Trump supporter, vowed on Sunday to work to defeat Ryan, the House speaker, in his Aug. 9 Wisconsin primary race against a conservative businessman.

Speaking on CNN's "State of the Union" program, Palin criticized Ryan for saying he was not yet ready to endorse Trump.

"Paul Ryan and his ilk, their problem is that they become so disconnected from the people they are elected to represent as evidenced by Paul Ryan's refusal to support the GOP front runner that we just said, 'He's our man.'"

Ryan said last week that conservatives wanted to know if Trump shared their values. He said he hoped to eventually support Trump but added, "I'm just not there right now."

Trump's tax proposal, released last September, included broad tax breaks for businesses and households. He proposed reducing the highest income-tax rate to 25 percent from the current 39.6 percent rate.

Pressed on the contradiction between his latest comments on taxes and the September tax plan, Trump said that he expected his original proposal was a "a concept" and he expected that it would be changed following negotiations with Congress.

"By the time it gets negotiated, it's going to be a different plan," Trump told ABC. He emphasized in interviews with both ABC and in a separate interview with NBC's "Meet the Press" that his priorities were lowering taxes on the middle class and businesses.

"The middle class has to be protected," Trump told NBC. "The rich is probably going to end up paying more."