Meaning of caucus-race in Alice in Wonderland?

Alice wins a thimble in the caucus-race. ...she must have a prize herself, you know,' said the Mouse.

Of course,' the Dodo replied very gravely. What else have you got in your pocket?' he went on, turning to Alice.

Only a thimble,' said Alice sadly.

Hand it over here,' said the Dodo.

Then they all crowded round her once more, while the Dodo solemnly presented the thimble, saying We beg your acceptance of this elegant thimble'; and, when it had finished this short speech, they all cheered.

Alice thought the whole thing very absurd, but they all looked so grave that she did not dare to laugh; and, as she could not think of anything to say, she simply bowed, and took the thimble, looking as solemn as she could.