A competitive U.S. Senate race in Hawaii is a once-in-a-generation occurrence.

And that's exactly what may be shaping up as two Democrats, Rep. Mazie Hirono (may-ZEE' hee-ROH'-noh) and former Rep. Ed Case, square off in a primary Saturday.

Each one wants to take on former two-term Republican Hawaii Gov. Linda Lingle in November.

Whichever Democrat wins is in for a tough fight from Lingle.

The outcome also could help determine the balance of power in Washington. Republicans need to pick up four seats to regain control of the Senate.