A constitutional expert says Prime Minister Stephen Harper's support of the NDP plan to hold a nationwide referendum on abolition of the unelected Senate is likely little more than political posturing.

Errol Mendes, a constitutional law professor at the University of Ottawa, told CTV News that Layton and Harper "know this will not get through the Senate."

"So, again they want to use the Senate as a stick to beat the Liberals with in the election."

NDP Leader Jack Layton recently described the Senate as an outdated institution that has no place in a modern democracy.

Insiders say Harper is prepared to support an NDP motion that would call for a national referendum on Senate abolition at the time of the next general election that is set for October 2009.

Tory insiders say the prime minister will have the Conservatives vote for the NDP motion that could be tabled in the Commons as early as next Tuesday.

The NDP motion would put pressure on Liberal Leader Stephane Dion to either support the plan or side with his unelected colleagues. Dion says getting rid of the Senate would open up a constitutional can of worms, something he won't support.

Political historian David Mitchell says it's more likely that the Senate will remain as is, referendum or no referendum.

He told CTV Newsnet: "To get the constitutional amendment with the agreement of the majority of the provinces, seven provinces, and to get the House of Commons and the Senate -- a Liberal dominated Senate -- to agree would be a very, very high threshold for approval, probably impossible."

But Layton claims there is provincial support to abolish the Senate.

"Quite a few premiers have already said that they support the idea of the abolition of the Senate -- from all parties," Layton told Mike Duffy Live.

"Certainly, the Bloc Quebecois and its platform called for the abolition of the Senate. They said it was undemocratic and wrong to have this kind of institution.... To vote on laws, you should have to be elected and that's the problem with the Senate, you have unelected people that are appointed by prime ministers who are long gone."

Quebec's Liberal government has indicated it's against any plan for abolition, and the Atlantic provinces enjoy favourable representation in the Senate. But premiers of four provinces -- Ontario, Manitoba, Saskatchewan and British Columbia -- have all called for abolition of the upper chamber.

The NDP referendum plan is similar to an idea floated by Conservative Senator Hugh Segal.

Harper put forward his own reform proposal that call for electing senators but that has been blocked by the Liberal-dominated Senate.

Harper has also put legislation before Parliament that would set eight year terms for senators.

The combined Tory-NDP vote would allow for the passage of Layton's proposal but it would still likely have to pass the Senate, where Liberal senators have objected to any significant reform.

The NDP motion would put pressure on Liberal Leader Stephane Dion to either support the plan or side with his unelected colleagues.

The Senate consists of 105 members, appointed by the Governor General on the advice of the prime minister. Seats are assigned on a regional basis, with each region receiving 24 seats. Senators must be at least 30 years old, and they can serve until they reach the age of 75. They earn more than $100,000 a year, not including pensions and benefits.