Democrats do hold back on sanctioning Iran. There have been many examples in the past history of politics, where democrats have sanctioned other countries for far less than anything Iran has done. This may not seem like the right thing to do, but Iran holds power with their oil. Without Irans oil, the prices for oil products would be much higher than they are now.
We have enough oil to be living in such oil fear from the middle east. If sanctions should be brought upon a country it shouldn't matter if they have something we want. That sets a bad precedent for how international relations work. Oh they get a pass because they have "x" resource we want, but this country doesn't have anything we need so we will cut them off.
Politicians are unwilling to risk a rise a gas prices at a time when people are happy with prices at the pump. Iran is a bully and a supporter of terrorism. If they did not have oil, the sanctions would be passed very easily. Some of our representatives need to grow a pair.
Going back as far as I can remember, cynical commentators mistaking themselves for insightful analysts have been reducing the entirety of US foreign policy to the significance of a single commodity: oil. But could anything really be so simple? The fact is that there are incredibly many factors that influence US foreign policy, and oil is only one of them, and not even the most important.