Federal opposition leader Thomas Mulcair tours the Stampede grounds during the 100th Anniversary of the Calgary Stampede in Calgary, Alberta, July 12, 2012. (REUTERS/Todd Korol)

OTTAWA - NDP Leader Thomas Mulcair argued Wednesday that only his party — and not the Liberals or any other — is capable of replacing the Conservative government of Stephen Harper.

It's an argument long used by federal Liberals to steal away potential NDP voters, the idea that the so-called "progressive" vote must unite under one Liberal banner in order to defeat Conservative candidates.