When talking about the diplomatic victory, you can not emphasize population growth too much. Play Lal, run democratic and say, FM or planned, not Green, even if it would ruin your everlasting friendship with Deidre
Beeline to the Empath guild, which by the way <i>does not multiply your votes</i>. I thought so myself before, but then I carefully checked, and it increases the votes with 50 %, so instead of 2 votes to 4, you get 2 to 3.
Ah, I had played for a year yet this mistake had remained.

Clinical Immortality proves it's utter usefulness, since at that tech level you have usually well enough votes and psych to manage without that extra talent. And the Telepathic Matrix, remember, is closing in...
Having many Pact brothers/sisters, preferrably subjugated, are your way to victory, unless you want to do it the nice way, by not crossing anyone (Can be hard) and reproducing like small mammals :naughty:.
The only problem in war waging is that your infrastructure and thus wealth and pop will be lower = getting more votes by buying them gets more difficult, and add to this the fact that you betray and kill others on the way.
So, there is the way of war and peace. I myself have found peace a better way, though I rarely go for the diplomatic victory at all.