Of a political, lobby, or religious group, to declare a "big tent" is to attempt to build a broad coalition of otherwise diverse groups around one central common concern, in order to amass sufficient support to influence others. This central shared concern or belief is the metaphorical tent pole around during which adherents can gather.
Most modern political parties in North America have this nature. New single-issue parties may spring up but they usually struggle for traction, and may find their message co-opted by the established groups. In Europe the alliances are more fluid and groups may splinter and reform around different tent pole issues.
In Canada the political right fractured in the mid-80s over ideological issues, collapsing the federal big tent of the Progressive Conservative Party of Canada(1). Eventually the right merged again and the 'big tent' reformed as a route back to power, but shifted to the right on the spectrum as the no-longer 'progressive' Conservative Party of Canada. Whether the reformed power block is a positive development is a matter of debate, but it did produce three successive governments under Stephen Harper.
It was widely expected that the U.S. Republicans would experience a similar schism after a Hillary Clinton win in 2016, but when that victory failed to happen, the shoe switched to the other foot and the Democrats are in disarray. Those hoping for a realignment and more parties are doomed to disappointment, though. As long as the U.S. version of a first past the post electoral system continues, the big tent parties will probably remain firmly in control.
Those hoping for clowns of a non-political persuasion should consult big top instead.
- In particular, the Reform Party of Canada and the follow-on Canadian Alliance took the rightmost part of the tent away.