WASHINGTON (AFP) - A US judge Monday ruled that a key provision in the Obama administrations landmark health care reform requiring all Americans to have insurance was unconstitutional. It was the first major legal blow to President Barack Obama concerning the radical overhaul of the nations health care system which he has made a cornerstone of his administration. On careful review, this court must conclude that Section 1501 of the Patient Protection and Affordable Care Act specifically the Minimum Essential Coverage Provision exceeds the constitutional boundaries of congressional power, the federal judge said in his ruling. Nancy-Ann DeParle, director of the White House office on health reform, played down the ruling, pointing to the other cases pending on the constitutionality of the law. This is one of 20 and we have already prevailed in two others, she told CNN. We believe the law is constitutional. We believe it is constitutional to say that everybody needs to be in the system, that everybody needs to have health insurance if they can afford it, if they cant they get help doing it. She added: The lawyers at the justice department they will be making decisions, and making a recommendation of how we move forward. Republicans have also vowed to try to roll back the reform when they formally take control of the House of Representatives in January, after making major gains in the November elections. Youll see us move quickly enough, leading Republican John Boehner promised last month. He is set to replace Democratic Representative Nancy Pelosi as speaker. Republicans believe Obamas health care overhaul will bankrupt our nation and believe it needs to be repealed and replaced with common-sense reforms to bring down the cost of health insurance, he added. Rolling back the sweeping measure was a rallying cry for US conservatives, especially the Tea Party movement that helped power massive Republican gains in November 2 elections.