Remember: the Democrats are the Wall Street party, and the GOP is the Pentagon party. Those are the primary interest groups that control each party, based on their enormous leverage to make things go from good to bad if they don't get what they want, one in a financial way and the other in a security way.
The Pentagon wants to weaken Iran because they are a historically powerful nation in the region, and threaten to upset the existing balance of power, whereby the US supports the jihadist states of the oil-rich Gulf, their terrorist proxies throughout the region, and bringing Israel along as a sidekick (no oil). Once the Iranian Revolution removed themselves from the list of client states of Uncle Sam (under the Shah), the Reagan and Bush administrations targeted them for weakening. That went somewhat dormant under Clinton, but reached another fever pitch under Bush Jr. (they were part of the "Axis of Evil" speech). That fell dormant again under Obama, and has picked up again under Trump's Pentagon-controlled foreign policy.
Recall that Trump himself has always preferred detente with Iran and wants to make deals with them. But between a total political neophyte with minimal political capital, and the institution that controls the armed forces, that view has lost out to the standard neocon BS.
Why were Clinton and Obama relatively less hostile toward Iran, including the major deal that the Obama team led to get them to reduce their nuclear program in exchange for removal of economic sanctions? The Democrats are controlled by Wall Street, who is not interested in playing the geopolitical chess game -- they just want to make shitloads of money from whoever they can, however they can. Iran is a nation of 80 million people, stable compared to other Middle Eastern countries, increasingly prosperous, and sitting on a ton of oil wealth that could be spent on consumption of foreign goods and services.
This early after the sanctions have been lifted, most of the foreign companies doing business in Iran are manufacturers, and not big banks -- but give it time. Here is a review from the WSJ a couple months ago:
After years shunning Iran, Western businesses are bursting through the country's doors -- but U.S. companies are noticeably absent.
Dozens of development projects and deals have been hammered out since Iran's nuclear accord with world powers in 2015 lifted a range of sanctions. Among them, France's Peugeot and Renault SA are building cars. The U.K.'s Vodafone Group PLC is teaming up with an Iranian firm to build up network infrastructure. Major oil companies including Royal Dutch Shell PLC have signed provisional agreements to develop energy resources. And infrastructure giants, including Germany's Siemens AG, have entered into agreements for large projects. ...
Government-approved foreign direct investment shot up to more than $11 billion last year, official figures show, from $1.26 billion in 2015. Pedram Soltani, the vice president of Iran's Chamber of Commerce, said more than 200 foreign business delegations have visited Iran since the nuclear deal took effect.
So far most of the foreign investors are from Europe or China, with the US still too anxious to get deeply involved. Probably because the European governments are not so heavily committed to antagonizing Iran in the geopolitical game, meaning less risk for those investing over there. The American companies must always be worried if the Pentagon party wins the White House and starts targeting them for doing business in the country that the Pentagon most wants to weaken and contain.
American manufacturers may not be able to participate so much, but the Wall Street money men are globalist in outlook, and just want to open up Iran to foreign direct investment already, and the Americans get to join whenever they get to join.
The half-baked view is that Obama sought the Iran deal because he's a Muslim-lover whose main goal was apologizing for past American imperialism. In reality, he was a figurehead whose entire Cabinet was hand-picked by Wall Street, and they just wanted access to a new market. In the half-baked view, Trump is Mr. Muslim Ban hell-bent on undoing the Iran deal -- in reality, his Pentagon overseers just gave hundreds of billions in arms to the custodians of the Two Holiest Cities, and only want to contain Iran for geopolitical reasons.
More and more policies start to make sense when we see the Democrats as the Wall Street party, and the Republicans as the Pentagon party. It also helps us make sense of Trump before he got boarded by the Pentagon, back when he was just a commentator or candidate. He was kind of a Republican but also kind of a Democrat -- wanting to open up Iran for American companies, though presumably with greater deals for manufacturers than banks, and with as much of that investment consisting of finished goods sent over there rather than locally manufactured, to help out American workers.
As long as the Pentagon is aligned with the dissolute and nearly bankrupt jihadist Gulf nations, though, American workers will never get their products sold into large, stable, and relatively prosperous Iran.