Donald Trump's presidential campaign this week went through more gyrations following the candidate's own inflammatory comments.

The week of ugly discourse ended with Trump trying to back away from the most incendiary assertion yet: Calling President Barack Obama and Democratic presidential nominee Hillary Clinton the "founders" of the Islamic State terrorist group.

Later, when conservative radio talk show hosts gave Trump a chance to clarify the comment, he made it clear.

"No, I meant he's the founder of ISIS," Trump responded when one host reminded him that Obama wants to destroy Islamic State forces. "I do. He was the most valuable player. I give him the most valuable player award. I give her, too, by the way - Hillary Clinton."

Trump did backpedal, in his own way, finally on Friday suggesting on Twitter that the media failed to recognize his "sarcasm." But he muddled his message later in a speech in Pennsylvania, reiterating that he was "being sarcastic," then added: "But not that sarcastic, to be honest with you."

Trump's shoot-from-the-hip style had raised eyebrows with a comment Tuesday that many interpreted as a suggestion gun enthusiasts should use violence against Clinton.

Trump kicked off the week trying to refocus his campaign on his economic vision, making more sharp criticism of US trade policy and calling for cuts in taxes and government regulations to spur investment and increase jobs.

He stayed on script, speaking to the Detroit Economic Club, but had to pause multiple times while police escorted protesters out of the room.

Clinton criticized his plan as one that would give "super big tax breaks to large corporations and the really wealthy, just like him," then released her own tax documents and demanding he do the same.

While Trump's mouth ran throughout the week, so did more Republicans - away from the controversial candidate - while poll numbers showed his support wilting in the August heat.

Senator Susan Collins of Maine announced she would not vote for Trump, citing a "constant stream of cruel comments and his inability to admit error or apologize." The four-term senator said she believed having Trump as president would make "an already perilous world" even more dangerous.

Senate Majority Leader Mitch McConnell was another top Republican who took a dim view of Trump's verbal antics, saying he hopes Trump "settles down and follows the script."

McConnell sounded the alarm about the effect of Trump's candidacy on Republicans who face tough battles in the November 8 election, openly worrying that the conservative party could lose its majority in the Senate.

Trump's troubles with his own party were magnified by a letter reportedly being drafted urging the party's chairman to cut off funding to Trump's campaign. More than 70 Republicans had signed the letter, according to news reports, which quoted it as saying Trump's "divisiveness, recklessness, incompetence and record-breaking unpopularity risk turning this election into a Democratic landslide."

They said the party should spend its money to help Republicans running for Congress to "prevent the GOP from drowning with a Trump-emblazoned anchor around its neck."

New poll numbers released Friday showed how support for Trump is evaporating in four swing states - Colorado, Florida, Virginia and North Carolina.

Clinton is ahead by 8 points in Colorado (43 per cent to 35 per cent) 7 points in Florida (44-37), 6 points in North Carolina (44-38) and 9 points in Virginia (44-35), according to the poll conducted by NBC News, The Wall Street Journal and Marist College Institute for Public Opinion.

When Trump was asked about other poll numbers showing support for him in decline, he said he would not abandon his confrontational style. He said he still believed he would win the election, but indicated the thought of losing had crossed his mind.

"Look, all I do is tell the truth," he said, and if on November 8 if he falls short due to his political incorrectness, "it's OK. I go back to a very good way of life."

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