It'll be a smaller "dent", if there is any at all. It's easy to determine how much you spend. It's a lot tougher to determine how much you'll receive.
Trying to reduce the deficit by raising taxes is a whole lot riskier than doing it through spending cuts.
Say what you will about Romney, but he was absolutely right about one thing conceptually: If you have to borrow money to give someone a Obamaphone, it probably shouldn't be borrowed.
Really it doesn't matter how much you soak the rich, you'll never cover the spending gap. That problem is far bigger than the "fair share" "problem".