WASHINGTON, March 2 (Reuters) - The U.S. Supreme Court on Monday agreed to hear a politically explosive case on whether Obamacare is lawful, taking up a bid by 20 Democratic-led states to save the landmark healthcare law.

The impetus for the Supreme Court case was a 2018 ruling by a federal judge in Texas that Obamacare as currently structured in light of a key Republican-backed change made by Congress violates the U.S. Constitution and is invalid in its entirety. The ruling came in a legal challenge to the law by Texas and 17 other conservative states backed by President Donald Trump's administration. (Reporting by Lawrence Hurley; Editing by Will Dunham)