Thanks to Roger Chesley at the Virginian-Pilot for covering the "firehouse primary" method of nominating candidates. Since we'll probably be seeing a few of these this year, we'd like to clarify the process when it comes to Democratic nominations for House of Delegates races.
District nominating committees determine the method of nomination. A nominating committee, per Article 7 of the DPVA Party Plan, is made up of the chairs of the Democratic committees of the localities that are within the House of Delegates district. If the chairs don't live in the district, they appoint someone from their local committee who does. The nominating committee determines whether or not nominations will be made by primary, assembled caucus ("mass meetings") or unassembled caucus ("firehouse primary"). For House of Delegates nominations, we typically see primaries or firehouse primaries.
Primaries are run, governed, and paid for by the government. You vote at your local precinct polling place, and the election is open to everyone - regardless of party identification. A caucus, on the other hand, is operated and paid for by the Party.
Firehouse primaries are technically unassembled caucuses. Instead of voting at your normal precinct polling place, precincts are combined such that there are only a few polling places in the district (as few as two). The locations and hours of operation of these polling places can advantage/disadvantage candidates, and there's usually a candidate filing fee ($500-$3000) to help pay for the operation of the election. Some, if not all, of that fee is often refunded afterwards. In order to vote in a caucus, you usually have to sign a pledge indicating you're a Democrat and intend to vote for the Democratic candidate in the general. If your voter history shows you've been active in Republican primaries, don't expect to waltz into a Democratic firehouse primary undetected.
Whether or not any given candidate benefits from a primary or caucus depends very much on the dynamics of the specific race and district. In general, however, Mr. Chesley is correct - "firehouse primaries" are inherently exclusive and therefore less democratic than primaries.
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Example Call to Caucus for Firehouse Primary.