Greg Selinger (right), new leader of the Manitoba provincial NDP and premier-designate, is welcomed into the premier's office by outgoing premier Gary Doer at the Manitoba legislature on Sunday. (Canadian Press)

Opposition leaders in Manitoba are warning the grace period for incoming premier Greg Selinger could be a short one.

"I think the honeymoon will be brief to non-existent," said Hugh McFadyen, leader of Manitoba's Progressive Conservative party.

Premier-designate Greg Selinger will be officially sworn in as Manitoba's 21st premier in a ceremony Monday.

On Saturday, delegates of the provincial New Democratic Party selected Selinger as their new party leader and premier.

Selinger defeated fellow candidate Steve Ashton by a wide margin.

The leadership vote was necessary after Gary Doer announced Aug. 27 that he was stepping down. The next day, he said he was accepting an invitation to become Canada's next ambassador to the U.S.

Doer's first day on the job is Monday.

McFadyen said Manitoba has enjoyed a healthy economy and generous transfer payments from the federal government in the past decade that the NDP under Doer has held power.

But Selinger, finance minister since 1999, faces a different reality, McFadyen said.

"He's in a position now where it looks like federal transfers are going to be reduced," said McFadyen. "The provincial economy is basically at a standstill right now."

Provincial Liberal Leader Jon Gerrard said Selinger's appointment marks a shift to the political left that may have economic consequences.

"Under Greg Selinger we will see a move towards a tax and spend approach," said Gerrard. "Look out for deficits developing."

Both opposition leaders said they agree with Selinger's promise to bring the legislative assembly back into session before Christmas and have an election two years from now, in 2011.

Speaking after his win Saturday, however, Selinger appeared upbeat about the NDP's future in Manitoba.

"We are renewed," said Selinger. "We are energized. We are ready to seek the future and take on the opportunities that it provides."