Republican politicians have campaigned on repealing President Barack Obama's healthcare reforms pretty much since they were enacted in 2010.
Now, with a governing majority, they've had to come up with a replacement plan - a task that has proved much more challenging than they may have imagined.
Here's a look at some key differences between the existing law, informally known as Obamacare, and the American Health Care Act, crafted by the Trump administration and Republican leadership in the House of Representatives.
Obamacare: All Americans are required to have health insurance or pay a tax penalty.
Republican plan: The mandate is repealed, but individuals who forgo health insurance for more than 63 days must pay a 30% surcharge on their insurance premiums for a year.
Obamacare: Companies with more than 50 employees are required to offer health insurance or pay a penalty.
Republican plan: This mandate is repealed.
Obamacare: Raised Medicare taxes on the wealthy and imposed new taxes on medical devices, health insurers, drug companies, investment income, tanning salons and high-end health insurance plans.
Republican plan: Repeals most Obamacare taxes and delays implementation of the tax on high-end health insurance plans to 2026.
Insurance for dependents
Obamacare: Required insurers to allow children under age 26 to be covered by their parents' policies
Republican plan: Maintains this requirement.
Pre-existing condition coverage
Obamacare: Prohibits insurers from denying coverage or charging more to individuals who have pre-existing medical conditions.
Republican plan: Gives states the option to waive pre-existing coverage requirements if they set up high-risk insurance plans for individuals who would be priced out by traditional insurers.
Essential health benefits
Obamacare: Requires all insurance plans to cover certain health conditions and services, such as emergency room visits, cancer treatment, annual physical exams, prescription drug costs, mental health counselling and women's health services.
Republican plan: Allows states to define what benefits are mandated or opt out of the requirement entirely.
Obamacare: Expanded Medicaid health insurance for the poor to cover more low-income individuals.
Republican plan: Curtails federal funding for Medicaid expansion beginning in 2020 and give states the option to receive federal dollars as a block grant instead of based on number of enrolees.
Obamacare: Prohibits insurers from limiting the amount they would reimburse for expenses for "essential health benefits" and curbs the annual amount a family must pay for these benefits.
Republican plan: Continues this prohibition and retains limits on so-called out-of-pocket expenses.
Obamacare: Provided refundable tax credits for low-income individuals who purchased their insurance on government-run marketplaces and support for some out-of-pocket medical expenses.
Republican plan: Makes tax credits based on an individual's age, which are phased out at higher income levels. Ends tax credits for out-of-pocket expenses.