OTTAWA – Alberta United Conservative Leader Jason Kenney says he regrets personally attacking Prime Minister Justin Trudeau, and is apologizing for doing so.

Last week Kenney called Trudeau "empty," having "the political depth of a finger bowl," and incapable of digesting "a briefing note longer than a cocktail napkin," in the Calgary Sun.

"I regret that, it wasn’t the style I aspire to, so I apologize to the prime minister for making personal comments," Kenney said in an interview with Don Martin, host of CTV's Power Play.

Kenney said since becoming leader he’s made an effort not to get into "ad hominem attacks," and he did last week, but won’t in the future.

He pledged to drop the personal attacks, but went on to say that there are “plenty” of issues of substance he can take on, adding that Trudeau is “the most unpopular prime minister in history in Alberta, in polling history.”

"That is the level of public frustration right now, so while I will avoid personal criticism in the future, I make no bones about the fact that I believe this prime minister’s reelection would not be in the interest of Alberta."

Asked to comment on Kenney’s remarks, Trudeau said on May 17: "It is not my practice and it is not I think helpful, to make personal attacks or to denigrate an individual. My father raised me better than that, and I'm not going to engage."

That day, according to a Canadian Press report, Kenney said he was standing by his comments.