A merger between the NDP and Liberal Party is the only way for them to form a government, the head of the Canadian Autoworkers union says.

CAW head Ken Lewenza is praising NDP MP Pat Martin, who promises to run for the NDP leadership if no other candidate supports looking into a merger or alliance with the Liberals. Martin says there should be at least one candidate in the race who is open to the idea.

Most NDP and Liberal MPs, including Liberal Interim Leader Bob Rae, have dismissed the idea. But Lewenza says anyone who thinks about the issue logically rather than emotionally will see it's the only way to beat the Conservative Party in the next election.

"The reality is, if you want to govern, you have to take a look at who's been governing Canada for the last decade. And the Liberals prior to the Tory majority government took advantage of a Conservative Party in division," Lewenza told Evan Solomon in an interview on CBC's Power & Politics.

"It's really time in Canadian politics to talk about uniting centre-left policies, centre-left public policy initiatives."

"It's simple numbers. It's simple fact. This is not rhetoric. At the end of the day, if you an combine the left-of-centre votes into one political party, you have an opportunity to govern," Lewenza added.

He admits there are policy differences between the Liberals and NDP, but says an alliance or merged party could deal with the differences in the interests of Canadians. He says Prime Minister Stephen Harper will use their differences against them if they don't formally unite.

"Egos have to be put aside ... The reality is it'll be controversial no matter what you call it," Lewenza said.

Lewenza says he isn't guaranteeing support to any candidate on a merger, but will look at which leader is most in the interest of progressive politics.

The NDP have long held union support. Jack Layton, the party's late leader, made his first public speech after the May 2 election to a national union gathering. The party filibustered back-to-work legislation for Canada Post for several days in June in a debate that ran through the night and into the weekend.

The NDP federal council will meet next week to set the rules and date for the upcoming leadership race to replace Layton, who died of cancer last month. One question is whether they'll give extra weight to affiliate members like the unions, whose votes counted for 25 per cent of the total ballots in the party's 2003 leadership race.

The party's constitution uses a one member, one vote formula but the federal council can set additional criteria if they choose.