Why is Obamacare so controversial?

After a troubled rollout and despite widespread unpopularity, President Barack Obama's 2010 healthcare law is forging full-steam ahead.

The Patient Protection and Affordable Care Act, known as Obamacare, is the largest overhaul of the US healthcare system since the 1960s.

What does the law do?

Its aim is simple: to extend health insurance coverage to some of

the estimated 15% of the US population who lack it. Those people receive no coverage from their employers and are not covered by US health programmes for the poor and the elderly.

To achieve this, the law requires all Americans to have health insurance, but offers subsidies to make coverage more affordable and aims to reduce the cost of insurance by bringing younger, healthier people into the health insurance system.

It also requires businesses with more than 50 full-time employees to offer health coverage, although this provision was delayed until 2015 to allow more time for compliance.

The law creates marketplaces - with websites akin to online travel and shopping sites - where individuals can compare prices as they shop for coverage.

In addition, the law bans insurance companies from denying health coverage to people with pre-existing health conditions, allows young people to remain on their parents' plans until age 26, and expands eligibility for the government-run Medicaid health programme for the poor.

The law aims eventually to slow the growth of US healthcare spending, which is the highest in the world.