Republican candidate Donald Trump has taken a thinly veiled swipe at Hilary Clinton’s husband Bill Clinton over his history of infidelity.

Details of Mr Clinton’s sexual relationship with White House employee Monica Lewinsky were revealed in 1998, with the then-president denying he had an “inappropriate” relationship with Lewinsky.

Mr Trump hinted at the scandal during the first presidential debate last week, saying he would not mention Bill Clinton out of respect for his opponent.

But Mr Trump discarded the subtle approach during a rally in Pennsylvania on the weekend.

Ms Lewinsky and Mr Clinton at a White House function in the 1990s. Photo: Getty

The Republican candidate accused Ms Clinton of single-minded selfishness.

“Hillary Clinton’s only loyalty is to her financial contributors and to herself,” Trump told the crowd.

“I don’t even think she’s loyal to Bill, if you wanna know the truth.”

“And really, folks, really, why should she be, right? Why should she be?” he said, in a clear reference to the former president’s indiscretions.

Mr Trump also encouraged chants of “lock her up” during the rally, in reference to Ms Clinton’s email scandal, as well as questioning her mental stability.

“She could be crazy. She could actually be crazy,” he yelled.

“She’s never done anything meaningful. Never.”

“Her only legacy is death, tremendous financial loss, and failure. Her policies gave us the disasters in Iraq, Libya, and Syria.”