Upheld again by the US Supreme Court on Thursday, President Barack Obama's landmark health care law has survived numerous political and legal challenges since it was passed by Congress more than five years ago.

This "never-ending saga," in the words of liberal justice Elena Kagan, has been a catalyst for conflict between the Democratic president and his Republican adversaries since the 2008 presidential elections.

Here are key dates along the way:

— March 23, 2010: Obama signs the Affordable Care Act, passed by Congress two days earlier along strictly partisan lines. It fulfills Obama's campaign promise to extend affordable health insurance coverage to 30 million uninsured Americans.

The same day, 13 US states led by Florida file suit to contest the law's constitutionality. They are joined later by 13 other states.

— January 31, 2011: The 26 states go before a judge in Florida, who strikes down the law, ruling that its requirement that every citizen obtain health insurance, or pay a penalty, is unconstitutional. The Obama administration appeals.

— September 28, 2011: After several conflicting court rulings, the administration asks the Supreme Court to decide whether the law is constitutional. The 26 states also appeal to the top US court, asking it to overturn the law.

— November 14, 2011: The Supreme Court agrees to hear the petitions by the government, the 26 states and a business organization.

— March 26-28, 2012: The court hears six hours of arguments for and against the law over a three-day period, a record.

— June 28, 2012: The top court upholds the law, while placing limits on the expansion of for the poorest Americans.

— October 1, 2013: The government's Healthcare.gov website, set up to allow people to sign up for private plans, has a chaotic debut, bedeviled by system crashes and low participation.

— October 2013: Republicans go on the attack in Congress, demanding that law's implementation be delayed and paralyzing the federal budget in the process.

— January 1, 2014: The law officially goes into effect, but 34 states refuse to establish the insurance exchanges envisioned by the law as state-sponsored marketplaces where individuals can compare and buy insurance plans offered by private companies.

— April 2014: The proportion of uninsured Americans falls from 16 percent (44 million) in 2013 to 13 percent.

— June 2014: The Supreme Court deals a setback to the , ruling that no employer is bound to pay for contraception in employees coverage plans if it is contrary to their religious beliefs.

— November 7, 2014: The Supreme Court takes up a complaint brought by four Virginia residents on the question of tax subsidies for the newly insured.

— February 2015: The House of Representatives votes for the 60th time to repeal the Affordable Care Act, but the Senate does not follow suit.

— March 4, 2015: The Supreme Court holds another hearing on the Affordable Care Act. Sixteen million people who formerly were uninsured are now covered.

— June 25, 2015: The Supreme Court upholds a bitterly opposed portion of the act, ruling that tax subsidies can be applied nationwide.