South Carolina Sen. Lindsey Graham didn't care for Donald Trump's comment at an NBC candidates' forum Wednesday in which he said Russian President Vladimir Putin has been more of a leader than American President Barack Obama has.

"This calculation by Trump unnerves me to my core," Graham told reporters on Capitol Hill Thursday, according to The Washington Post and other media outlets.

Graham was one of Trump's 16 challengers for the GOP presidential nomination that was won by the billionaire New York real estate developer and reality TV star. He was a vocal critic of Trump during the primaries, but has softened his tone since Trump won the nomination.

Trump has expressed admiration for Putin in the past after the Russian leader praised him, and Graham was unhappy with the latest comparison of Putin and Obama, even if Obama is from the opposing party.

"Vladimir Putin is a thug, a dictator, an autocratic ruler who has his opposition killed in the streets in Russia," Graham said. "If you are running to be leader of the free world and you find admiration for Putin, well, then, you're losing me."

Graham agreed that Obama has been "weak" and "indecisive" against Putin, adding that the Russian leader has "walked all over him. But no, I'm not gonna say that Putin's a better leader than the Democratic elected president of the United States, even though I have differences with him."

Graham posted on Facebook that he wasn't happy with either Trump's or Democrat Hillary Clinton's appearances at the forum.

"If you were looking for leadership, you didn't find it in Hillary Clinton's performance. If you were looking for competency, Donald Trump fell short," he wrote. "Simply put, I've never been more worried about our men and women in uniform being poorly led over the next four years."