Orlando - After days of alleging repeatedly that US President Barack Obama literally founded the Islamic State group, Donald Trump abruptly shifted tone on Friday and insisted his widely debunked claim had been sarcastic.Trump, in an early-morning post on Twitter, blamed CNN for reporting "so seriously" that he had called Obama and Democrat Hillary Clinton the extremist group's founder and most valuable player. He added, in all capital letters: "THEY DON'T GET SARCASM?"Only hours before, the billionaire businessman had restated the allegation with no mention of sarcasm, telling rally-goers in Kissimmee, Florida, that "I've been saying that Barack Obama is the founder." It's a claim that Trump repeated at least a dozen times in three cities since debuting the attack-line on Wednesday during a rally outside Fort Lauderdale.In fact, Trump had refused to clarify that he was being rhetorical or sarcastic when asked about the remark during interviews. On Tuesday, when conservative radio host Hugh Hewitt tried to steer Trump toward explaining he really meant Obama's Mideast policies created conditions that ISIS exploited, Trump wanted none of it."No, I meant he's the founder of ISIS. I do," Trump said, using another acronym for the extremist group. Told that Obama was trying to defeat the militants, Trump added, "I don't care. He was the founder."Worrying signsThe controversy over the Islamic State has dogged the campaign in a week in which he has been trying to highlight his economic proposals. Trump is encountering worrying signs as his campaign moves into the November election.Clinton's lead over Trump in national polls has widened in recent days, while a growing number of fellow Republicans have declared they won't support their own party's nominee.It wasn't immediately clear why Trump altered course on Friday and said the whole notion was sarcastic. But the allegation had elicited fresh concerns about Trump's relationship with the truth and his preparedness to be commander in chief.Trump has blamed Obama's decision to pull US forces from Iraq in 2011 for destabilising the Middle East and creating a situation in which Islamic State militants could thrive.He'd added Clinton to the mix by noting her initial support for the Iraq War and her ties to Obama's policies as his first-term secretary of state. However, Trump previously had said he wanted US troops out years earlier than Obama withdrew them.The founder of the Islamic State group was Abu Musab al-Zarqawi, the head of al-Qaeda in Iraq who was killed in a US airstrike in 2006. The group began as Iraq's local affiliate of al-Qaeda, the group that attacked the US on September 11 2001.GOP concerns about Trump are compelling enough that dozens of worried Republicans were gathering signatures for a letter urging the party's chairman to stop helping Trump and focus on protecting vulnerable House and Senate candidates.Trump said he wasn't worried Republicans would cut him off - and threatened to stop fundraising for the party if they do.