The Affordable Care Act requires every citizen to purchase health insurance. They can purchase the insurance through state run exchanges or from federal exchanges. Since some Americans cannot afford insurance, the government provides subsidies based upon their income. It is fairly obvious that the ACA was not designed to restrict subsidies to state run exchanges and to deny them to citizens that use the federal exchange. The US Supreme Court believes otherwise. It has agreed to hear a case in which the claimants argue that citizens who use the federal exchange are not entitled to subsidies. If the Supreme Court decides for the claimants, the Republican Party would have achieved what it was unable to achieve through legislation. They will have effectively repealed the ACA. Subsidies would only be available through state exchanges and GOP controlled states have not developed exchanges for their citizens. Citizens from GOP controlled states who are entitled to subsidies would no longer be able to purchase affordable healthcare insurance.
The Republican majority in the Supreme Court has decided to hear the case because Republican justices in state courts have ruled that the intent of the law was to restrict subsidies to state run exchanges. That interpretation would require the court to argue that the law, which only works with subsidies for those with low incomes, was not intended for all Americans. Such an interpretation is only possible in court system that has been politically corrupted.