NASHVILLE (AP) — Gov. Bill Haslam won't endorse any of the Republicans seeking the nomination to succeed him, but said he has spoken to several potential candidates about the emotionally taxing nature of a running a statewide campaign in Tennessee.

Haslam was the subject of heavy criticism from his rivals for the GOP nomination when he ran for his first term in 2010.

His three opponents attacked him early and often on a range of topics, including his refusal to divulge earnings from the family-owned Pilot Flying J truck stop chain; his former membership in the national group Mayors Against Illegal Guns; a long-ago donation to Democratic U.S. Sen. Al Gore to help retire the Tennessean's debt from his first presidential run in 1988; and for what they called his likelihood of supporting a state income tax.

"Almost without exception I've had a conversation with everybody about what it's like to campaign for two years, most of that in a primary when you're in a friendly battle," Haslam told reporters at the state Capitol this week.

"It's a physically grueling process that has a certain personal vulnerability to it that takes most people awhile to get used to," he said.

Haslam was the most prolific fundraiser of the 2010 race, and also ended up pouring $4.25 million of his own money into the race to seal the deal against state Senate Speaker Ron Ramsey of Blountville and U.S. Rep. Zach Wamp of Chattanooga. Former Memphis prosecutor Bill Gibbons dropped out earlier in the nomination fight.

Wamp aired television ads targeting Haslam as a "billionaire oil man," claiming that Haslam would be a "puppet" for his father and brother at Pilot and welcome the introduction of a state income tax. Ramsey hammered away at Haslam for what he called his suspect support for Second Amendment rights.

"Haslam has all the money he wants, so we're fighting a machine gun with a Derringer," Ramsey complained at the time. "So we're going to have two shots, they better be up close."

Even after Haslam won 47 percent of the Republican primary vote — compared with Wamp's 29 percent and Ramsey's 22 percent — some bitterness remained.

"The future of this party and this country is not in Nantucket," Wamp said at a GOP unity event following the primary, an apparent reference to the Massachusetts island where the winning candidate's brother, Jimmy Haslam, had a vacation home. Bill Haslam was quick to point out to reporters that he didn't "hang out there."

The declared Republican candidates for the August 2018 primary include Haslam's former economic development chief Randy Boyd of Knoxville, Franklin businessman Bill Lee and state Sen. Mae Beavers of Mt. Juliet. U.S. Rep. Diane Black of Gallatin and state House Speaker Beth Harwell of Nashville are considering joining the race.

Senate Majority Leader Mark Norris of Collierville, who had been considering a bid, has not yet commented on news reports that he is being vetted for a nomination to a seat on the federal court bench.

Haslam insisted that he's staying out of the primary fight.

"I am an interested observer like everyone else," he said. "I do not have a favorite."