Controversial author Ed Klein, a former editor of New York Times magazine, is traveling in Indiana with Republican front-runner Donald Trump.

The author confirmed to the Washington Examiner on Monday that he is with Trump as he campaigns around the state ahead of Tuesday's primary, which is expected to be a tough battle against the billionaire's nearest rival Ted Cruz.

"We're moving from place to place," said Klein. He declined, however, to give details about why he is with Trump.

"I'm actually in the middle of doing something right now, unfortunately, but I can't talk about it now or for the rest of the day," he said.

Klein is the author of the 2015's Unlikable: The Problem with Hillary, a critical book that purports to deliver an insider account of Democratic candidate Hillary Clinton and her long quest to become president. It depicted Clinton as a power-hungry politician who lacks the charisma and charm to fulfill her White House ambitions.

Klein maintained in his book that Clinton is severely ill and possibly physically unfit to be president.

In late April, Klein published a column at Newsmax explaining that he has a relationship with Trump. "I've known Trump for 35 years, met with him on numerous occasions, talked to him on the phone countless times, traveled with him, and written two lengthy magazine cover stories about him," he wrote.

He said Trump has "an ace in the hole" to lock up the GOP presidential nomination.

Before the rise of Trump, however, Klein believed it was Marco Rubio who would be the best bet to defeat Clinton in a general election matchup. Klein recommended John Kasich as Rubio's best choice for a running mate.

A spokesperson for the Trump campaign did not return a request for comment.