McNaughton's withdrawal left just Barrie MP Patrick Brown and deputy PC leader Christine Elliott in the race, and he's backing Brown, a backbencher who chairs the federal Conservatives' Greater Toronto Area caucus.
Elliott is supported by the same party establishment and backroom operatives that allowed the Liberals to win four consecutive elections, while Brown has "the energy and ability" to reform the Tories, McNaughton said.
"He's 36. I'm 38, and I think the choice is clear for people: it is the same old party establishment versus change," he said. "I don't want to see the people that have been running campaigns for the last 20 years run the campaign in 2018. We will have another devastating result."
McNaughton, who was a distant third in membership sales and fundraising, said he and Brown have similar views on how to rebuild the once powerful party.
"I believe that we need an agent of change in the PC party," he said.
Brown, who doesn't have a seat in the Ontario legislature, also criticized Elliott as the favoured candidate of the Tory establishment.
"This leadership contest has become what we always knew it was: a choice between the same old, same old, and a fresh start to rebuild our party," he said.
Elliott, the widow of former finance minister Jim Flaherty, said she wasn't surprised McNaughton threw his support to Brown, saying both were representative of the American "Tea Party" style of thinking.
"They are much more on the right-wing side of the party and socially conservative, and I think really out of touch with mainstream Ontario," she said.
Elliot said she offered a "positive, pragmatic and truly progressive conservative vision," and called Brown "an untested candidate with nothing more to offer than a life lived as a career politician" who has no substantive record.
Brown, who was first elected to Barrie city council while attending university and later elected to Parliament at age 27, surprised everyone by selling over 41,000 PC memberships.
But the Elliott campaign says his support is concentrated in a small number of ridings while hers is broadly based across the province.
The ballots will be weighted so each of the 107 ridings gets 100 votes for the leader, so where memberships were sold is just as important as how many.
All PC members will be eligible to vote for the leader May 3 and 7, with the results to be announced at a convention in Toronto on May 9.
The Brown and Elliott campaigns have been trading barbs for weeks, with Brown claiming Elliott's team used phone calls to harass some of his supporters while Elliott accused Brown of hiding how much money his campaign has raised.
"This has been a trend since it became apparent that we had a membership lead of up to 3-to-1 on Ms. Elliott," Brown said of the attacks from his rival.
The latest postings with Elections Ontario show Elliott raised $780,000 in donations, compared with $752,500 for Brown and $118,000 for McNaughton.
There had been five candidates, but North Bay MPP Vic Fedeli and Nepean-Carleton MPP Lisa MacLeod withdrew earlier and threw their support to Elliott.
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