In a comment that lit the Twittersphere and torqued up Missouri’s ongoing ideological tax debate, conservative mega-donor Rex Sinquefield told a Washington audience Thursday that Gov. Jay Nixon was “an idiot” for trying to stop a state income tax cut.

He repeated the word later in the speech, but by the next day was backing off from it. On Friday, amid growing condemnation from the media and the public, his spokeswoman issued a statement saying he “regrets his personal comments about Gov. Nixon.”

However, even in that apology, Sinquefield doubled down on his core argument: that tax cuts would spur economic growth, and that the Democratic governor was thwarting it.

“[Sinquefield’s] remarks were born out of frustration,” said the statement from spokeswoman Laura Slay. “He thinks that it is unfathomable that our state leader vetoed the modest tax reform measure, which is a positive move in the right direction for our working families and small businesses.”

Nixon’s office didn’t respond Friday to requests for comment.

Sinquefield’s comments came during a speech to the conservative think-tank The Heritage Foundation. In it, he compared Nixon unfavorably to Kansas Gov. Sam Brownback, a Republican who has championed deep tax cuts in that state.