This is very costly to do. Listen to a political candidate's speech, then try to validate what they are saying on your own. It's incredibly time consuming.

A ratings service that relies on the person being rated to pay results in conflicts of interest. You will also find it hard to have anyone want to pay for anything that might be critical of them.

A service that did this would require a strong guiding mission, because it's cheaper to shill blog spam than to actually dig in and do accurate research while citing sources.