Donald Trump has said several times that he wants the Affordable Care Act (Obamacare) to fail. Unfortunately, there are some things he can do to make insurance harder to get or less affordable. He's already started doing it.
The law strives to make health insurance more affordable for people who have to buy their own. One is to give all most of them tax credits to help them pay the cost of premiums. (The credit is enough to limit their cost to a percentage of their income, based on the cost of the the least expensive silver plans available where they live.) Another is to give low-income working people help them copays and deductibles that most of them otherwise couldn't afford. This latter part is called "cost-sharing reductions" or "CSRs."
Some opponents of the Affordable Care Act claim that the wording of the law does not expressly appropriate funding for the CSRs, though when the bill was originally analyzed by the Congressional Budget Office and by independent organizations, and when it was debated in Congress, everyone on all sides of the issue assumed that the appropriations were automatic.
President Trump now claims that he has the right to withhold the payments. He has so far not tried to do so, but month after month, he suggests that he might decide to cancel the payments next month. This obviously puts insurance companies in a bind, since the law says they have to cut what they charge, and if the federal government breaks its promise and doesn't pay, the companies are on the hook for the money and will have to increase premiums next year.
This has already created a crisis in Nevada, where Anthem Blue Cross Blue Shield has elected not to offer individual health insurance policies next year in all but the state's largest counties. Since no other insurers are likely to offer health insurance in most or all of these counties, this is a serious problem. (For more, see for example this article in The Reno Gazette-Journal.)
This isn't the only way Trump can screw with the Affordable Care Act. Last month, health insurance blogger Charles Gaba worked out, based on a sample of insurance company rate increase requests from 20 states, that while more than a quarter of the projected premium increases in 2018 can be blamed on such things as the general increase in medical costs, fully 71 percent is attributable to politics, specifically actions (or inaction) by the administration and Congress. Gaba isn't a neutral observer -- in fact, he makes it pretty clear that he's pissed off at the people he deems responsible -- but that doesn't make him wrong.
For more, see this video from pediatrician and medical school professor Dr Aaron Carroll: