big tent

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A group, especially a political coalition, that accommodates people who have a wide range of beliefs, principles, or backgrounds.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic A group, philosophy, or social or political movement that encompasses or seeks to attract a broad range of members or constituents.
• noun attributive, sometimes hyphenated Pertaining to, representing, or advocating such a group, philosophy, or movement.