"The party is ready for prime time and I just think it needs a different leader to get it to the next level," the five-term Edmonton member of the legislature said at a hastily called news conference Tuesday.
"We need to ... go beyond our traditional supporters so that there is a strong and very effective group of MLAs in the next legislature, after the next election, that will stand up to the Wildrose and possibly to the PCs as well."
Mason said he will resign as head of the party effective Oct. 19. That same weekend party faithful are to meet to pick a replacement.
He indicated he will stay on as an MLA for Edmonton-Highlands-Norwood and is likely to run again in the 2016 election.
Mason, 60, said there was no one thing that prompted his decision. It was something he'd been mulling for awhile.
He suggested Alberta is at a crossroads politically as the right-of-centre Wildrose party presents a formidable challenge to the governing fellow right-centre Progressive Conservatives.
"But much like the PCs before the last election, the Wildrose is now pretending to be a moderate and socially progressive political party," he said.
"However, there are hundreds of thousands of Albertans who can see through this deception. They are looking for a new political home.
"A leadership race and a new leader are the best means to attract those progressively minded Albertans to our party."
Mason didn't refer to Raj Sherman's Liberals, but intimated that party doesn't have the infrastructure or financing to keep up.
He said it's the NDP that "has the political, organizational and financial capacity to stand up to the Wildrose and the Progressive Conservatives in the next election."
Last week, Alberta's chief electoral officer deemed 53 Liberal riding associations defunct for failing to provide financial statements, meaning they can't raise funds or solicit candidates.
Recent fundraising figures show the NDP raised almost $124,000 in the first three months of this year compared with about $80,000 for the Liberals.
Mason's decision plunges Alberta into a second leadership race. The Progressive Conservatives officially launch their race May 15 to replace former premier Alison Redford. Voting is to take place Sept. 6 and, if necessary, a final ballot will be held Sept. 20.
Mason's team has work to do.
The New Democrats have just four sitting members in the 87-seat legislature, all in Edmonton.
The party captured almost 10 per cent of the vote in the 2012 election but struggled to gain traction with voters outside the provincial capital.
Mason's fellow caucus members — Dave Eggen, Deron Bilous and Rachel Notley — were at his announcement but declined to comment on leadership aspirations.
"This is (Mason's) day," said Notley.
Eggen called Mason "a mentor" and added: "He's by far the best legislator in the house.
"He's professionalized the Alberta New Democrats like nobody has in several generations."
There were plaudits from politicians of all stripes, including Premier Dave Hancock.
"I am proud to call Brian a friend and have enjoyed having him as a political sparring partner since our days in university," said Hancock in a news release.
"I know that in the government caucus, Brian Mason is held in high respect. He certainly has mine."
Mason, a former transit bus driver, took over leadership of the provincial New Democrats in 2004.
He served on Edmonton city council for 11 years before making the jump to provincial politics in 2000 when he won a byelection.
He said his greatest legislature triumph was keeping public pressure on former premier Ralph Klein to force him to back away from further privatization of health care under his "third way" plan.
Fellow politicians also lauded Mason's trademark wit that has skewered many an opponent on the chamber floor.
It was on display again Tuesday.
"Are you going to take a break from the legislature? Are you going to be absent?" he was asked.
"Yes," Mason deadpanned. "I'll be in Palm Springs."
He was referring to recent photos on social media sites of fellow MLA Redford dining and cycling in the California resort city while the legislature is sitting.
When the laughter died down, Mason cleared the air.
"No," he said. "I'll be on the job."
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