One of the original national finance chairs for Mitt Romney has revealed he will be voting for Hillary Clinton.

In an op-ed for CNBC, David Nierenberg, who is now president of a Washington, D.C.-based investment management company, reveals he has been a Republican voter every presidential race (except one) since 1980.

But this Election Day will be different.

"[Donald] Trump is the most dangerous major party presidential candidate in my lifetime, maybe in American history," Nierenberg, who along with Meg Whitman was one of the former Massachusetts governor's first national finance chairs, wrote. "His character, temperament, and behavior definitely are not presidential. I don't think he's fit to be our president. He speaks positively about foreign dictators and acts like one himself."

Nierenberg, like his former boss, took issue with Trump's failure to release his tax returns.

"For decades, candidates, including successful business people, have released their tax returns; why does Trump think he shouldn't follow the rules of the game? Defeating him has to be our national priority," Nierenberg lamented.

Though he noted the other ballots he will cast in November will all be for "real" Republicans, he has decided to endorse the Democratic nominee in Clinton.

Clinton "knows her stuff," he said.

"She is emotionally mature and centered. She respects and enjoys working with people from all backgrounds. She has the diplomatic skills needed to break the gridlock in Washington and lead our country well. America needs a steady hand on the tiller," Nierenberg concluded. "We cannot afford the risk of a man whose temperament and behavior are erratic."