OTTAWA -- Conservative leadership hopefuls made some last-ditch pitches for support Monday in advance of a key campaign deadline -- and stanching the flow of illegal migrants across the Canada-U.S. border played a prominent role.

Candidates only have until Tuesday to sign up new party members in order to buttress support for their leadership bids, so several -- Kevin O'Leary, Maxime Bernier and Steven Blaney were doing their best to grab some Tory-friendly headlines.

In an early email blast, businessman and reality-TV star Kevin O'Leary suggested using the Constitution's veto power, known as the notwithstanding clause, to block refugee claims from people who enter Canada illegally.

It's a response to a recent spike in illegal border-crossings from the U.S. into Canada, in many cases by people apprehensive about a Trump administration that's limiting immigration and stepping up deportations.

Under the Canada-U.S. Safe Third Country agreement, migrants who seek to cross at official border crossings are turned back if they have already made a refugee claims in the U.S.

Ottawa could act unilaterally if the U.S. doesn't agree to amend that agreement, O'Leary suggested.

"If the U.S. won't agree to closing the loophole so that those crossing into Canada illegally are returned to the U.S. where they are entitled to a fair hearing before an independent court, then Canada must act on its own," said the email.

Earlier Monday, former cabinet minister Bernier sent a basic solicitation, urging would-be supporters to sign up before 5 p.m. ET Tuesday.

But he later doubled down on O'Leary's message, saying not only would he use the notwithstanding clause, but he'd deploy the military as a "temporary" measure to backstop existing police and border resources.

"If it takes too long to put that additional manpower in place, I will look at additional temporary measures -- including deploying Canadian Forces in troubled border areas," Bernier said in a statement.

Steven Blaney sent out an eyebrow-raising fundraising email of his own on the weekend, denouncing alleged anti-Semitic remarks by some Muslim leaders.

The email, containing the subject line "Should Allah kill all the Jews?", referenced individuals including an imam and former Ryerson University teaching assistant who was recently removed from his position over alleged anti-Jewish remarks.

Leadership hopeful Michael Chong was also making headlines Monday -- but not for his campaign efforts.

Social media was abuzz all day about a Globe and Mail item from March 22 by columnist Leah McLaren, since taken offline, in which she describes trying to breastfeed Chong's infant baby during a house party more than 10 years ago, "just to see what it felt like."

On Twitter, Chong shrugged it off as "no doubt odd, but of no real consequence."

As the deadline loomed, other would-be Tory leaders opted for the straightforward approach, highlighting the support they've already drummed up through new memberships or from within the party's office holder ranks.

Pierre Lemieux reinforced his opposition to legalizing marijuana amid news that the Trudeau Liberals planned to introduce legislation this spring to legalize pot by July 2018.

Lisa Raitt posted a video on her social media accounts, urging Canadians to join her under a "big blue tent."

And Erin O'Toole's campaign sent out separate emails announcing more endorsements, including a pack-leading 26 sitting MPs who were publicly backing his leadership bid.

Divergent messages, but all with the same goal, said Mainstreet Research president and CEO Quito Maggi.

"These are all attempts to differentiate themselves from the other candidates," said Maggi, whose firm has been tracking the leadership race through rolling surveys.

While the party said it doesn't expect to release official numbers until some time in April, Maggi said he anticipated roughly 175,000 individuals paid the required $15 membership fee to become eligible to vote for a new leader, with that number likely to approach 200,000 by the deadline.

Just over 251,000 people took out memberships in 2004 before Stephen Harper was elected to lead the party, newly formed by a merger between the federal Progressive Conservatives and the Canadian Alliance, said party spokesman Cory Hann.

Once past Tuesday's deadline, the candidates will turn to encouraging voter turnout -- and in the case of so-called "top tier" candidates, work back channels to shore up support from candidates who may quickly fall off the ranked ballot, said Maggi.

"Even if someone has signed up 10,000 or 15,000 people, the most important thing is getting those people to actually vote," he said.

"But now it's (also) about trying to shore up that second choice."

The party is set to choose a permanent replacement for Ambrose on May 27 through a preferential ballot.