Republican Senator Lindsey Graham on Saturday vowed to investigate President Donald Trump’s claim that former President Barack Obama ordered the wiretapping of his phones during the election, saying he was "very worried" by the claims.

Speaking from Clemson University in his home state of South Carolina, Graham said he did not know whether or not Trump’s accusation — which the president presented, without evidence, in a tweetstorm Saturday morning — was true.

“I don’t know if it’s true or not, but if it is true, it would be the biggest political scandal since Watergate,” he said.

“The other side of the story, if the former president of the United States was able to obtain a warrant, lawfully, to monitor Trump’s campaign for violating the law, that would be the biggest scandal since Watergate,” he added.

A spokesman for Obama, Kevin Lewis, on Saturday said neither the former president nor White House officials ever ordered surveillance on a US citizen.

Sen. Graham told the crowd he was “very worried that our president is suggesting that the former president has done something illegally,” before adding that he would be “very worried if, in fact, the Obama administration was able to obtain a warrant lawfully about Trump campaign activity with foreign governments."

"So it's my job as a United States senator to get to the bottom of this. I promise you I will,” he said.