Republican Donald Trump on Friday backed away from comments calling President Barack Obama and Democratic nominee Hillary Clinton the founders of the militant group Islamic State, while the Republican Party sought to project unity behind their candidate.

A new poll showed Trump, whose unfiltered speaking style has repeatedly landed him in hot water, losing ground in three crucial states ahead of the Nov. 8 general election against Clinton.

In a surprise appearance, Republican National Committee Chairman Reince Priebus, who in private expressed fury over some of Trump's actions earlier this month, introduced the candidate at a campaign event in Erie, Pennsylvania, and the two hugged onstage.

"We’re so honoured to be working with Donald Trump and the campaign," Priebus told thousands of Trump supporters.

"And don’t believe the garbage you read. Let me tell you something. Donald Trump, the Republican Party, all of you, we’re going to put him in the White House and save this country together."

Republican sources earlier this month said Priebus was furious over Trump's failure to endorse House of Representatives Speaker Paul Ryan and his feud with the parents of a Muslim American soldier killed in Iraq. Trump did endorse Ryan a few days later.

Trump brought Priebus on stage later at another rally, in Altoona, Pennsylvania, to thank him for the work he has done as he insisted there is great unity in the party.

"I have to say we have great unification," Trump said.

Trump on Friday told the rallies in Altoona and Erie that his remarks earlier this week calling Obama and Clinton the founders of ISIS, as Islamic State is also known, had been sarcastic.

"I have been saying because it's true, but somewhat sarcastically, that he's the founder of ISIS and she's a close second," Trump said in Altoona.

Trump first made the unfounded claim on Wednesday and repeated it through the week.

Trump claimed sarcasm in July as well after he was heavily criticized for inviting Russia to dig up tens of thousands of "missing" emails from Clinton's time as U.S. secretary of state.

A Wall Street Journal/NBC News/Marist poll released on Friday suggested support for Trump is eroding among voters in three battleground states.

Such states are hotly contested because their populations can swing either to Republicans or Democrats and thus play a decisive role in presidential elections, which are ultimately decided by the state-by-state tally of the Electoral College.

The poll found Clinton widening her lead in Colorado, Virginia and North Carolina, while holding her advantage in Florida.

Clinton released her tax returns on Friday, painting the move as a sign of transparency that her campaign says Trump lacks.

U.S. presidential candidates are not required to release their tax returns, but it has become a common custom.

Trump has cited an audit by the Internal Revenue Service in refusing to release his returns. Trump also has said his taxes are no one's business and that they reveal little.

Trump scheduled a speech in Warren, Ohio, on Monday that will focus on how he would handle the threat posed by Islamic State. Trump has said he would "knock the hell out of ISIS," without offering details.

Trump has been mired in repeated controversies in recent days. He drew heavy criticism after he suggested gun rights activists could take action against Clinton, a statement he later said was aimed at rallying votes against her.

Nearly one-fifth of registered Republicans now want Trump to drop out of the race for the White House, according to a Reuters/Ipsos poll released on Wednesday.

Republicans frequently trace the birth of Islamic State to the Obama administration’s decision to withdraw the last U.S. forces from Iraq by the end of 2011.

But many analysts argue its roots lie in the decision of George W. Bush’s Republican administration to invade Iraq in 2003 without a plan to fill the vacuum created by Saddam Hussein’s ouster. It was Bush’s administration that negotiated the 2009 agreement that called for the withdrawal of all U.S. forces from Iraq by Dec. 31, 2011.

At his Erie event, Trump seemed to acknowledge he is facing a formidable opponent in Clinton as well as a difficult electoral path.

"The Republicans have a tougher path – not my fault,” he said.

He said Clinton's campaign is smart to keep her out of the spotlight.

"She doesn’t talk to reporters very often. ... She doesn’t expose what’s going on up here, which isn’t good,” he said, meaning her brain. “She’s doesn’t expose her mind to questions. What they want to do is try to fake it through.”

Trump also said in Altoona that the only way he could lose Pennsylvania to Clinton is if "cheating goes on."

He said he wants authorities to monitor the voting closely. "I know what's happening here, folks. She can't beat what's happening here."