Yes, most definitely it helps the cause. Displaying a bumper sticker is almost as good as a $250 contribution. Making a donation and displaying the bumper sticker is even better.
I have talked to people who fear their car will be vandalized if they display a McCain bumper sticker. That in itself tells me to vote for McCain.
If it didn’t help, they would do it. When you see a bunch of someones signs, you feel like they have a lot of support and must be well liked, it might sway a person’s vote. Not an educated voter, but one of those people who decide on the day of election kind of voter.
Years ago, I put a sign up when I lived in NC.. My neighbor saw it, and voted for the person on my sign. She commented to me that she saw my political sign, and thought she would vote the same as me “since I know a lot..”