Nervous investors sent the dollar lower on Tuesday ahead of what many expect to be a dovish message from the Federal Reserve Wednesday, as new data shows the "long dollar" is three times more crowded than any other trade.

The Fed is widely expected to raise the Fed funds rate by a quarter point Wednesday for the first time since June 2006, while emphasizing that it will be slow to hike rates further. But any hints it could raise rates less gradually could boost the dollar.