True, OTOH, the President sets the budget
Congress may add, deduct, play with the figures, but I doubt they'd go too far afield since the President is sitting there with veto power. They negotiate everything to death before they actually finalize the budget and send it back to the President for signature. If he slashes NASA, I doubt they could add too much back. If they did, he might negotiate away something Congress wants badly. I may be wrong but the President has a lot of influence, particularly if Congressional majorities are from his/her own party.
It's only one factor on my balance sheet as to whom to support.