Yes it is possible I've done it several times but I play on lower difficulty levels beginner/normal I have my tax rate at 31 and spending at 60 to increase morale and population growth and approval and normally when approval is at 100% on the following elections I get 100% candidates.I also only go to republic government not a democratic one although I do research the tech for the diplomacy increase.But its not all the time as I think theres a randomiser to take in galactic events into account.

I'd like to know that too. By not changing to a Federation he is losing out on a lot of money (40% bonus compared to 10% from the Republic).

Also, why is he reducing spending? The spending rate has no effect on the voting at all. The only reason I could see is, if setting his tax rate to 31% would bring him below the -500bc debt threshold, if he kept spending at 100%.