Saskatchewan MP Andrew Scheer has resigned as Opposition House leader in order to explore a bid for leadership of the federal Conservative Party.

He announced his resignation outside the party's caucus meetings in Halifax Tuesday morning, after informing his colleagues of his decision.

Scheer began his brief statement to the media in French, distinguishing himself from other leadership contenders who are not bilingual.

He said he had a wonderful summer travelling across Canada and consulting with party members about the race.

"I've been very encouraged by the feedback I've received and I'd like to continue to put together a team to continue exploring making a leadership run," he said, stopping short of fully declaring his candidacy.

Scheer thanked interim leader Rona Ambrose for the opportunity to serve, but said the House leader role requires 100 per cent of his attention, making it necessary to step down.

He said he would have more to say "once the final decision has been made and the final pieces have come together for a leadership bid."

'Unifying figure'

Parliament returns Sept. 19. The Conservatives are expected to announce a new House leader and shuffle their opposition critics accordingly later this week.

Before coming to Halifax, Edmonton MP Garnett Genuis told CBC News he would welcome Scheer's candidacy and he sensed a lot of caucus support for the former Speaker of the House of Commons.

"Scheer is a unifying figure for different parts of the Conservative family," he said, contrasting that with a candidate like Kellie Leitch, who introduced the wedge issue of screening for "Canadian values" into the campaign.

Fellow Alberta MP Shannon Stubbs said she was excited that Scheer might be getting in, saying that he was affable and likable and offered policy depth in tune with people's priorities.

She described him as a fiscal conservative and someone who could "bring people together."

MacKay's out, race wide open

Some potential leadership candidates may have been waiting to see what Peter MacKay did before jumping in. As of Monday night, they know he won't run, despite polling suggesting he was the front-runner.

Lisa Raitt, another senior caucus member who told CBC News last week she's made a decision but isn't ready to reveal it, still wouldn't confirm Tuesday whether she's in or out.

Lisa Raitt didn't answer questions about her leadership decision Tuesday morning. She said last week she knows what she'll do, but isn't prepared to reveal that yet. (Andrew Vaughan/Canadian Press)

Her former cabinet colleague Erin O'Toole also is still considering whether to join the race.

Ambrose said she had spoken to MacKay and told him to go home and kiss his wife. The door is open now for anyone to step up, she said.

Peter Van Loan said he's seen a lot of races where people lament the absence of big names but the eventual winner becomes prime minister or premier.

"The race is very young," said Quebec MP Gerard Deltell — one of many MPs who has yet to decide who to support.

Television personality and businessman Kevin O'Leary, who's musing about a possible run himself, told CBC News that MacKay's decision changes the dynamics.

"He was really sucking the oxygen out of the room because he was clearly going to be a big factor in the Ontario fundraising," O'Leary said, suggesting that MacKay's support had been as high as 50 per cent.

Now that MacKay's out, O'Leary believes his own support may grow. But he's also "interviewing" other candidates to see if someone else might be worthy of his endorsement. If not, he promises to enter the race himself.

Leitch controversy 'put her on the map'

O'Leary would need to wait until later in the fall when he meets the party's six-month membership requirement. (He only joined right before the party's May convention in Vancouver.)

But he could wait as late as February before jumping in, he said, because he's already well-known from television and could assemble his campaign infrastructure within 48 hours.

Asked about polling that suggested Leitch's support was rising since she started to talk about the need to screen immigrants for what they believe, O'Leary said "she was at two per cent in the polls" and needed to get her name out there.

"There's not a chance in hell that she's going to get my endorsement," he said, calling her ideas un-Canadian. "The chance that she would become leader of any party with that platform is zero."

Leadership hopeful Kellie Leitch, right, chats with Rob Batherson, chair of the Halifax Chamber of Commerce and an active Conservative outside the party's caucus meetings Tuesday. Leitch's comments about screening immigrants for Canadian values have divided party members. (Andrew Vaughan/Canadian Press)

But Immigration Minister John McCallum said in Ottawa Tuesday, "I don't think she was really significantly in the race until she made these statements and then that put her on the map."

Leitch stuck by her strategy Tuesday and said she was "delighted" to be gaining support.

"I am going to be leader in May of 2017 because I'm talking to members of our party about things they care about," she said.

Tributes for MacKay

Leadership candidate Maxime Bernier paid tribute to MacKay, saying that it was because of his decision to work with Stephen Harper and unite the two conservative parties in the first place that he was here.

"I think Peter can be proud of that," he said.

Bernier said MacKay's absence wasn't a cause for relief for his campaign because he got in the race to focus on his ideas, regardless of who else ran.

Erin O'Toole originally declared he wouldn't be a candidate for the Conservative Party leadership, but discussions with his caucus colleagues this summer have encouraged him to reconsider. (Andrew Vaughan/Canadian Press)

Tony Clement said he'd decided well before MacKay's decision to run for the leadership and feels confident about his candidacy.

O'Toole said that MacKay deserved to spend time with family and was "the best minister of national defence we had since World War II."

"​It's fitting that we're here in Atlantic Canada because he gave so much to Atlantic Canada," Raitt said. "What I cling to is he said that he's going to be there to help whoever the new leader will be in the future."