ST-BERNARD-DE-LACOLLE, Que. -- Andrew Scheer brought his national campaign to the edge of the U.S. border, where he pledged to shut a legal "loophole" that he says has enabled tens of thousands of asylum-seekers to cross north into Canada and claim refugee status.

The Conservative leader, however, didn't provide specifics Wednesday when pressed at a news conference to explain how he would get it done.

Scheer's biggest obstacle would be persuading U.S. President Donald Trump to renegotiate a treaty called the Safe Third Country Agreement.

The deal prevents asylum-seekers from claiming refugee protection in Canada if they arrive at an official border checkpoint from a country that is considered safe, such as the United States. The agreement, however, permits them to make such claims if they're already in Canada.

Scheer called it a "loophole" in the agreement.

In recent years, a very popular entry point for asylum-seekers has been Roxham Road, where southern Quebec meets the state of New York. A New York road dead-ends on the American side, separated from the Quebec road by a few metres of scrub between trees.

Scheer made Wednesday's immigration announcement at the end of Roxham Road, only metres from the frontier, in front of a command post that's been built for authorities to process illegal crossers.

He didn't say how he would renegotiate the agreement with the Trump administration, only that he would use unspecified "other tools" if he forms a government after the Oct. 21 vote.

"I believe it's in both countries' mutual benefit to have this situation resolved on a bilateral basis -- that is our preferred option," Scheer said in the community of St-Bernard-de-Lacolle, without explaining what the Americans would get out of a renegotiation.

"Of course, we will explore other options, but that is our first course of action."

The federal government says nearly 50,000 people have entered Canada at Roxham Road in the last two years, most of them intending to claim asylum once they're on Canadian soil, since President Donald Trump's administration has harshened the U.S. government's attitude toward refugee claimants.

The border issue has been a big concern for many Canadians, especially in Quebec. Scheer made the announcement on the eve of a French-language leaders' debate, the last debate before the Oct. 21 election.

Scheer argued that Liberal Leader Justin Trudeau had done "literally nothing over the past two years -- more than two years -- to resolve this issue."

The Conservative plan to stop the flow of so-called irregular border crossers includes hiring 250 additional border officers.

In his announcement, Scheer insisted there was a need to pay special attention to individuals who may be members of organized crime, such as MS-13. He was referring to the notorious El Salvador-based street gang, which Trump has also made a rhetorical target when he talks about immigration and border security.

Asked why he would single out MS-13 in his speech, Scheer said there have been reports of people coming into Canada with criminal records or who have been identified as members of criminal organizations.

"We want to make sure that when people come into Canada we are putting the safety and security of Canadians first," he said.

"That is paramount and people who have engaged in dangerous of violent activity in the past should not be in Canada."

Janet Dench, executive director of the Canadian Council for Refugees, said concerns about members of MS-13 are not relevant to Canada because federal numbers show that fewer than one per cent of refugee claims in 2018 were from El Salvadorans.

"This has been an issue raised by anti-immigrant politicians in the U.S.," Dench wrote in an email.

"Presumably Mr. Scheer is hoping to take advantage of the same xenophobic fears that U.S. politicians have been tapping into."

Dench added that Canada could easily solve the problem of irregular crossings by suspending, or withdrawing from the Safe Third Country Agreement. It would allow people, she said, to present themselves in an orderly way and be processed at a regular port of entry.

Former Conservative immigration minister Jason Kenney told Maclean's in 2017 that he asked the Obama administration to renegotiate the agreement and his American counterpart declined.

Scheer said a Conservative government would deploy more judges to popular crossing points over the shorter term to deal with a backlog of cases weighing on the system.

Overall, the Conservatives' immigration plan would also make it a priority to fund services such as language training, credential recognition and the protection of vulnerable people.

Canada's future immigration levels would be based on Canada's population needs and its economic needs, he said.

Scheer would not say whether that means allowing more -- or fewer -- immigrants into Canada.

"We determine a level based on data, based on information, based on the needs for our country," he said.

Earlier this year, the Liberals changed the refugee law to prevent asylum-seekers from making refugee claims in Canada if they've made similar claims in certain other countries, including the U.S., to prevent what the government has called "asylum-shopping."

Lawyers and advocates who work with refugees have said that was an attack on refugee rights in Canada.

Earlier Wednesday, Trudeau suggested Scheer's visit to Roxham Road would allow him to see the infrastructure that the Liberal government has installed in the area, where all new arrivals -- regular and irregular -- are held by the RCMP and fully screened.

"There are no shortcuts. There are no skipping steps within our immigration system. Everyone arriving in Canada goes through the same immigration system -- a full, rigorous immigration system that is being applied," Trudeau said.

This report by The Canadian Press was first published Oct. 9, 2019.