Yes, what is used in Portugal is "brandy" in the sense that it is
distilled alcohol produced from fermented grapes. It is fairly raw
stuff, about 70% to 80% alcohol. They use fermented grapes because
that's what they have available - wines that don't meet the quality
standards, and pomace after the press process. It isn't oak aged,
colored brown or watered down to 40% (80 Proof) like the "brandy" we
find in liquor stores in the States. It's "firewater" - a lot closer
to Everclear or moonshine than a commercial product.

I've used Bacardi 151 rum to fortify several batches of port-style and
sherry-style wines, with good results. The high alcohol level means
you are using MUCH less to achieve a given change in overall proof
(less dilution of flavor). The rum by itself has a very mild flavor -
it blends well with just about anything, and when used with port is
just not detectable - the stronger flavors of the port completely
dominate the mixture.

I've also used Everclear with good results, but less often, as I can't
buy it in Minnesota. A high-proof vodka would work about as well, I
expect. The two criteria I have are 1) high alcohol level, so as to
minimize dilution, and 2) no strong flavors that might clash with the
wine. So I wouldn't recommend gin, or tequila, or Scotch whiskey.
"Brandy" or bourbon whiskey would be a second choice, if I couldn't
get vodka or rum.

Doug