Prime Minister Stephen Harper's plan to stack 18 vacant Senate seats with Tory loyalists before Christmas was denounced by opposition MPs Thursday as a cynical move which contradicts past pledges to reform the upper chamber.

"This is just a hog-troughing orgy, it seems the Conservatives have tossed their principles out the window," NDP MP Pat Martin told CTV News on Thursday as word about the appointments spread over Parliament Hill.

The announcement was also seen as a sharp policy reversal for the prime minister, who has long been a champion of making the Red Chamber an elected body.

Liberal democratic reform critic Joyce Murray said the appointments "fly in the face" of the Tory election platform and will further erode the public's trust in the current government.

"This is not an act that will restore trust with Canadians, nor will it restore confidence in this prime minister in the eyes of Parliament," she said in a statement.

In the Tory throne speech, which was delivered les than a month ago, Harper said: "Our government believes that Canada is not well-served by the Senate in its current form."

Harper added in the speech that the Tories would continue to push for a new system that accounted for "direct consultations with voters on the selection of senators and limitations on their tenure."

But the Conservatives maintain that appointing the senators remains their prerogative, despite a Liberal - NDP threat to topple the government in the New Year.

Senior Tory staffers also said that Harper's hand has been forced because the Liberal-heavy Senate is dysfunctional.

Currently, there are 58 Liberals and 20 Conservatives in the Senate.

"It's an institution that needs to be functioning, and as we look at it now, it's not functioning in its full regime," said Foreign Affairs Minister Lawrence Cannon Thursday.

Industry Minister Tony Clement echoed those sentiments and stressed that the Tories haven't abandoned their promise of reform.

"We want a Senate that is elected, but clearly, the Liberal-dominated Senate does not want that," he said.

Liberal MP Wayne Easter, meanwhile, said that Harper has no right to make patronage appointments when the House of Commons remains locked in a constitutional grey area.

Easter noted that the majority of MPs don't have confidence in the prime minister, and that it is unacceptable that Harper "would turn around and stack the senate with his friends" after proroguing Parliament.

But according to a constitutional expert, Harper isn't out of line in making the appointments.

"Absolutely, unreservedly so, there are no limitations on his appointment powers," said Peter Woolstencroft, a political science professor at the University of Waterloo, in an interview with CTV Newsnet.

"The opposition will fume ... but he's not doing anything out of line."

Woolstencroft said that since the Tories could be thrown out of power next month, Harper is likely trying to plan ahead and bolster his party's standing in the Senate before it's too late.

"He wants to improve the odds."