The more you vote, the more of your voting power you will use up. A 100% strength vote with 50% voting power left is half as strong as when your voting power is fully charged.

There is really no harm other than it makes your votes worth less because you have less voting power.

If you vote excessively then eventually your voting power will get so low that it will not let you vote anymore until it recharges a bit (or you buy more SP).