NEW: Donald Trump says only way Hillary Clinton can win Pennsylvania "is if cheating goes on;" does not specify if he is being sarcastic. — NBC Nightly News (@NBCNightlyNews) August 12, 2016

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The tweet was an apparent reference to Trump’s explanation that he was using “ sarcasm ” when he repeatedly called President Obama the “founder” of the Islamic State in Iraq and Syria.

Trump trails Hillary Clinton by nine percentage points in Pennsylvania, according to a RealClearPolitics average of polls

"We're going to watch Pennsylvania. Go down to certain areas and watch and study and make sure other people don't come in and vote five times," Trump said at his Friday rally in Altoona, Pa.

"If you do that, we're not going to lose. The only way we can lose, in my opinion — I really mean this, Pennsylvania — is if cheating goes on."

The rally was attended by an estimated 3,000 supporters — the largest crowd ever at the Blair County Convention Center, according to Tribune Democrat newspaper.

No Republican presidential candidate has won Pennsylvania since George H.W. Bush in 1988.