A politician's "base" is the people who are his most solid supporters, the people that he can almost surely count on to vote for him, maybe even give him money, etc.
What the writer is saying here is that many of the products that China buys from the US are products that are made by people who support Mr Trump. For example they say that China buys a lot of US soybeans, and farmers tend to support Mr Trump.
I'd question whether the paragraph is accurate. I don't think auto workers are part of Mr Trump's base. I thought they mostly voted against him, but Mr Trump won Michigan because they didn't vote as overwhelmingly Democrat as they had in past elections, and so the union Democrat vote was eclipsed by the Republican small business and rural vote. But in any case -- and this is the relevant point -- auto workers are not part of Trump's "base" because they are not his core supporters. He picked up enough of them to win the election, but they are not solid, reliable votes for him in the future, and so would not be considered part of his "base".