That's the question of the day, now that a new article in the Washington Post indicates that the government is considering a package of business tax breaks to stimulate jobs ahead of the mid-terms, including a payroll tax holiday.
1) Practically, this isn't going to do anything before the mid-terms. These sorts of changes take time to roll out, and there's no way they could get anything into effect soon enough to make an actual difference in peoples' lives.
2) Whether you think this works as a campaign tactic depends on whether you think people will think that it is going to work. On that question, I have no idea. People do love cash in their pockets, however.
3) Politically, this has one major drawback: it's going to put huge holes in the Social Security and Medicare trust funds. Since I think those trust funds are meaningless accounting devices, I don't think this has any practical relevance. But as you will be able to see in my comment section about twenty minutes after I hit "post," people have a very deep emotional attachment to the idea of the trust funds, which politicians cannot easily trifle with.
4) Practically, I think the actual impact will be minimal, at least on employment. It might help people and companies to rebuild their balance sheets (or let struggling companies ride things out a while longer). But the main constraint on business hiring is uncertainty, and a payroll tax isn't going to change that. Obviously, some workers will get hired at the margin--but if your labor is so marginal that you need a payroll tax holiday to make it economical, then I'd expect that as soon as the payroll tax holiday is over, you'll probably be fired. Hence, even if you get the job, you're going to want to save as much of your wages as possible, blunting the multiplier effect we hope to get out of stimulus.