Here is some data on US import/export activity:
Exports
Machines, engines, pumps: US$205.8 billion (13.7% of total exports)
Electronic equipment: $169.8 billion (11.3%)
Aircraft, spacecraft: $131.1 billion (8.7%)
Vehicles: $127.1 billion (8.4%)
Oil: $106.1 billion (7.1%)
Medical, technical equipment: $83.4 billion (5.5%)
Plastics: $60.3 billion (4%)
Gems, precious metals, coins: $58.7 billion (3.9%)
Pharmaceuticals: $47.3 billion (3.1%)
Organic chemicals: $38.8 billion (2.6%)
Imports:
Electronic equipment: US$332.9 billion (14.4% of total US imports)
Machines, engines, pumps: $329.3 billion (14.3%)
Vehicles: $283.8 billion (12.3%)
Oil: $201.2 billion (8.7%)
Pharmaceuticals: $86.1 billion (3.7%)
Medical, technical equipment: $78.3 billion (3.4%)
Furniture, lighting, signs: $61.2 billion (2.6%)
Gems, precious metals: $60.2 billion (2.6%)
Organic chemicals: $52.1 billion (2.3%)
Plastics: $50.2 billion (2.2%)
In 2015 the net trade deficit for the US was 531 billion dollars. i.e. on a net basis we imported 531 billion more in goods and services than we exported.
What would a trade war look like?
Overall we would need to offset $532 billion mostly by increased cost of goods to US consumers. Then we wouldn't be at the mercy of US creditors like China.
1.
tax things we import . This means that for the consumer, things we are not good at producing would cost more. e.g. electronics... we import 333 billion in electronics but export only 170 billion. thus in electronics we have a trade deficit of 163 billion. If we tax imports in electronics, electronics will cost consumers more, causing them to buy less and pushing the US towards a more even balance of trade in electronics.
2.
Other countries will be forced to try to reset the balance by taxing things we export. e.g. aircraft. The US sells $131 billion in aircraft. Our import rate on aircraft is nil. So other countries will be forced to either tax aircraft purchases or change their sourcing of aircraft. A lot of the US sourcing of aircraft involves military planes (e.g. the F35), but a good deal is also commercial aircraft. When you talk aircraft you are talking Boing but there are other players including Airbus (a European consortium).
http://www.bga-aeroweb.com/firms/Competitors/Competitors-Boeing.html
Generating a trade war would mean increased pressure for countries to switch sourcing of aircraft away from the US.
I could go on with this but you get the idea. When we lose jobs for widget makers we trade those jobs for higher level jobs like Boeing engineers and manufacturing workers. If you are a US widget maker and are not taking part in the US economic success it's because you didn't listen to your high school counselors when they told you to go to college and become an engineer.
If you ARE an engineer you don't back Trump.
If you are not a college graduate and work making widgets you back Trump.
You are left behind... and now you want to solve your problem by electing Trump and resetting the US economy to favor widget makers. This MAY give you a temporary reprieve but only VERY TEMPORARY.
The problem is that there are so many people who have been left behind that it's possible they will win and give Trump the opportunity to fracture the US/world economy so we can make more widgets in the US, lay off engineers at Boeing and allow Airbus and other US competitors to grab high end market share.
The US does not compete at the widget level. This is a conscious choice because becoming a widget making country would make the US a third world rather than a first world country.