“How do you think U.S. citizens would react today if a similar war with similar causes/origins (like the War of 1812) was declared?”
Under similar circumstances, I suspect there would be a clamor for war from both Republicans and Democrats. I do not think, however, that there is much of an analogy to be drawn between modern crises and the War of 1812. It involved geopolitical issues that were very specific to the late eighteenth and early nineteenth centuries, and the United States exercises such a degree of strategic hegemony over the Western Hemisphere today that I do not think most comparisons would be valid.
I agree with the above posts who thought that we would probably go to war. If the issues were similar, like impressment and unfair seizure of goods in the War of 1812, we would definitely defend ourselves. Can you imagine the headlines if some country just started high-jacking our ships and oil tankers, for example? Oh my. With our media coverage the way it is, people in the U.S. would call for action. The U.S. has never appreciated being pushed around.
I think that we would certainly go to war in such a situation. We can even see proof of it in some ways in our war against the Taliban. In 1812, we fought the British partly because they were stirring up Indian attacks against our settlers. In 2001, we attacked the Taliban because they had allowed Al Qaeda to train on their soil to attack us. It's a pretty similar thing.
In the interest of full disclosure, I tend to vote liberally and Democrat.
Americans today wouldn't stand for the treatment of fellow American traders any more than their ancestors did. If another nation was aggressively impressing our men and women (forcing them to serve in an armed force) and/or taking our shipments of goods, our government would absolutely take action. We care too much about our economy, too much about our well-being, to let that kind of aggression go unanswered.
There would be pressure from Congress and the public to "Save America"; the far-right of the Republican party would doubtlessly seize the opportunity to strike fear in Americans while the far-left liberal agenda would likely stay relatively quiet. It's an issue both sides can agree on -- American goods and people should be protected -- but the noise of one side would drown out the solutions of the other. Our new War Hawks would push for a declaration of war and there probably would be one. The president, however, would likely want to steer the nation to a more evident victory; the war of 1812 was such a wash!
You might compare how the United States reacted to the kidnapping of more than eighty Americans during what historians call the Pueblo Incident with how the United States reacted to what historians call the Gulf of Tonkin Incident.
After the Pueblo Incident President Johnson negociated the release of the captured Americans. After the Gulf of Tonkin Incident, Presiden Johnson sent a half a million military personel into Vietnam.
Same president, competely opposite reaction.
I think the situation with Iran resembles the situation between the British Empire and the United States in 1812.
In 1812, the British very likely could have crushed the United States as if it were a bug if it had really wanted to do so. Oddly enough, if Iran has nuclear weapons, the same might be true now with Iran. I suppose it depends on the size of nuclear bombs. Can they be small enough for suicide bombers to carry?
In 1812, the United States and the British Emprire made nice. They wanted to make commerce more than the wanted to make war, so they negotiated a settlement.
One must wonder if that is possible with Iran.