It took me 23 months of long nights, with many fits and starts, but I am happy to say that I am finally done with merging three different GRP databases. I handed off the final merged data set off to the GRP Membership Director. Since I resigned from the State Committee and any of the other committees I was on, that concludes any direct involvement I have with workings of the Green-Rainbow Party at the state level. Both scary and exhilarating. New projects await.
My recommendation to anyone who maintains a party or non-profit database is to just have one. I recommend CiviCRM, though there are many others. If you must integrate other data sources, say a voter list on a periodic basis, then keep past versions of the data set around, extract the new records and merge those into your data set. Any conflicts will be easier to handle. If you do not have just one data source, then look at the brief notes I kept.