Former NDP candidate Michael Byers advocates for uniting the left.
There is only one surefire way to prevent a Harper majority. The Liberals and NDP should agree to not run candidates against each other in the next campaign. In each riding, the party whose candidate fared worst in the last election would pull its current candidate out, or refrain from nominating one. Both parties would win more seats, with the Liberals potentially forming a majority government.
Just last week, Eric at threehundredeight.com tested the results from the last election against a similar merger scenario. By his estimate, the Conservatives would have won exactly one less seat than they currently hold.