Mitt Romney for President
campaign: US President
current polls: 46% nationally
If you're a volunteer looking to get involved, going to the Romney campaign would be wise only if: 1) you're in a swing state, and 2) you have a lot of time to dedicate to the campaign. Now is a good time to get involved, because Romney is generally close or slightly down in the national polls. But because of the few days left in the campaign, you risk getting lost in the chaos and confusion of things.
If you are going to get involved, you want to find a few people you like and try to work closely with them. National campaigns often spur consulting groups and anyone working in a swing state is likely to have decent political connections. You want to get to know these people, make a good impression, and stay in contact.
When you walk in, be humble but be direct about any special skills that you have. They don't have three weeks to figure out what you're good at, so just get to the point. Realize that the Romney campaign is likely starved for minority outreach, so anyone with those capabilities would be enormously helpful. People in Ohio, Florida, Pennsylvania, and Wisconsin are also probably highly preferred.
You can call, email and signup as a volunteer on the website, but the best thing is always just to walk into the nearest office during normal business hours.