Once upon a time, in an America long gone, the Republican Club in Washington used to be in a standard Capitol Hill rowhouse on the block where the Library of Congress' Madison Building now stands. The Madison Building was built in the 1970's, after all the properties on the block had been acquired by the Architect of the Capitol, and razed. (A crime against historic preservation.) That left the Republicans at the time with a relocation decision: downtown vs. Capitol Hill.
The party leaders at the time recognized the logistical value of the Capitol Hill site, so the new Club was built next door to another new building housing the RNC. Both are relatively small, traditional brick buildings to fit in with the Capitol Hill historic district streetscape. These buildings are across the street from the Capitol South metro station and diagonally across the street from the Cannon House Office Building.
The location is wonderfully convenient for events. Since explicitly partisan activities are not supposed to take place in the Capitol itself or in the House and Senate office buildings, the GOP created its own venue right across the street. The convenient location boosts attendance; if you want a lot of congressmen to do a meet and greet, for example, hold your event at the Club, and they can just come across the street. Need to make some political (i.e. fundraising) calls? Run across the street, and use one of the rooms at the RNC or the Club. These are huge advantages for people as badly overscheduled as are most Members.
The Democrats grumbled for years that the GOP had stolen a march, and eventually opened their own headquarters and club a couple of blocks to the south. It is marginally less convenient, which is the penalty the dems paid for being late.
The Club is not "upscale." It has a perfectly adequate beer and burger casual dining room in the basement, and a very ordinary but serviceable formal sit down restaurant upstairs. As an event location, it is considered boring and definitely a low-cost choice, but everyone in the business values its convenience and, frankly, understands that routine fundraisers are about raising money, not dazzling guests with frills. The meeting rooms are also very heavily used. Some Members are regulars, as are many staffers, and rightworld lobbying types generally have memberships. It's casual, convenient, and moderately priced.
Thank you for the great history and tour!