Just doing their job. If the polls were honestly telling them that their Messiah is going to lose, they'd stay home.
And they know that a typical Republican this year (I am one) will do this: if the (honest) poll tells me Romney is going to lose - I'll go vote for Senator. If she is going to lose - I'll go vote for the House candidate. If he is going to lose - I'll go vote for local candidates. If they are all going to lose - I'll go vote against the newest sales tax increase scheme.
If all of my votes are "losers" - I'll go anyway, I just might get the chance to beat the c**p out of some union thug that's trying to keep me from voting.