Republican presidential nominee Donald Trump on Friday evening sought to end a high-stakes impasse with several members of his party's leadership by delivering a formal endorsement of House Speaker Paul Ryan's re-election after initially refusing to do so.

“We will have disagreements but we will disagree as friends and never stop working together toward victory. And very importantly, toward real change,” Trump said during a campaign event in Green Bay, Wis. “So in our shared mission to make America great again, I support and endorse our Speaker of the House Paul Ryan.”

Trump flashed two thumbs up and a smile as he made the formal endorsement.

He stirred a fight within his own party earlier this week when he pointedly withheld his endorsement from Ryan ahead of Tuesday's primary, angering many in the party's establishment wing and raising serious questions about whether he would be able to build a united front against his Democratic rival, Hillary Clinton.

Ryan was not at the Green Bay event, nor was Gov. Scott Walker.

Ryan has endorsed Trump, but he said Friday that his endorsement isn't a “blank check” and pledged to speak out against the businessman's divisive positions if necessary.

Trump acknowledged that he would need the help of various factions in the party during his campaign event Friday, also endorsing Arizona Sen. John McCain and New Hampshire Sen. Kelly Ayotte in their re-election efforts.

“I need a Republican Senate and House to accomplish all of the changes that we have to make. We have to make them,” Trump said. “I understand and embrace the wisdom of Ronald Regan's big tent within the party. Big, big tent, remember? Ronald Reagan. Great man. Great guy.”

Trump earlier Friday unleashed a scathing attack on Clinton's character.

At a rally in Iowa, he called Clinton “a dangerous liar,” “an unbalanced person,” “pretty close to unhinged,” “totally unfit to lead” and lacking “the judgment, temperament and moral character to lead the country.”

Associated Press contributed.