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Despite the nationwide interest, getting voters to the polls may be the key challenge in the off-year election. Mr. Huey is counting on the commitment of tea party activists, while Mrs. Hahn is expected to benefit from union get-out-the-vote efforts.

Polling for the race has been sparse, but the left-leaning Public Policy Polling released an election-eve survey giving Mrs. Hahn a 52 percent to 44 percent lead with 4 percent undecided.

That is outside the poll’s margin of error, but Mr. Huey already has surprised observers by making it to the runoff. The Democrat’s eight-point lead also is far below the 25-point margin Mrs. Harman ran up in her last election, and few consider Tuesday’s result a foregone conclusion.

“The whole thing is going to depend on turnout,” Mr. Stern said. “Republicans will win special elections that they ordinarily wouldn’t win because of turnout. Now, obviously, if both sides get out their base, then she wins, because Democrats have so many more voters.”