Why are campaign contributions legal?

The constitution specifically forbids elected office holders at the federal level from receiving any compensation, gifts or other enrichment other than their salary while in office. However, for some reason, we allow them to accept money to "finance their campaigns". Why? If they did not receive those contributions they would have to buy those ads with their own money. So campaign contributions are used to buy things that they would otherwise purchase for themselves. It helps them protect their personal wealth from the cost of campaigning. As such, they are in real danger of losing money if those donations don't come in. Then there are corrupt politicians like Janet Napolitano who appropriate campaign funds for personal enrichment through accounting tricks. I suggest that there is really no way to keep campaign funds separate from personal funds. One has far too much influence on the other. So how can political office holders fund their re-election campaigns without taking contributions that violate the constitution? I don't think they can. I don't think they should.

What I propose is not to outlaw campaign contributions in total. After all, those who don't hold public office can accept whatever money they want. So only those currently in office would be forbidden to collect contributions. I think this is fair and it would free up their time to actually do their jobs and insulate them from corruption. Basically an incumbant would either have to not run for consecutive terms or let their record speak for itself. Wouldn't it be great if our president wasn't on the road begging for money for half his first term? We wouldn't need term limits if we just held them to the standard that the constitution demands. While in office, you can't accept money or gifts of any kind whatsoever. I think it would have a positive effect on our government.