Five more states are joining a Florida-led group of states in a lawsuit challenging President Barack Obama's overhaul of the U.S. healthcare system, Florida's attorney general said on Wednesday.

The joint lawsuit led by Florida and now grouping 18 states was filed on March 23 by mostly Republican attorney generals.

It claims the sweeping reform of the $2.5 trillion U.S. healthcare system, pushed through by Democrats in the Congress after months of bitter partisan fighting, violates state-government rights in the U.S. Constitution and will force massive new spending on hard-pressed state governments.

While some legal scholars think the suits will reach the Supreme Court, many agree that the supremacy clause of the Constitution, which puts the powers of the U.S. government above those of the states, will trump the states' arguments.

"We welcome the partnership of Indiana, North Dakota, Mississippi, Nevada and Arizona as we continue fighting to protect the constitutional rights of American citizens and the sovereignty of our states," Florida Attorney General Bill McCollum said.