All the dbag responses saying that they'd take this as blackmail is exactly why you shouldn't do it. Either take the new offer or don't. If you want a raise, tell your boss why you deserve one and then ask for it or don't. Even if you ask in advance and nothing happens, then you show other offers, you're still screwed. You will get pushed out and treated badly by the dbags who are underpaying you anyway. Just move on...or don't.