(CNN) - Five more states - Indiana, North Dakota, Mississippi, Nevada and Arizona - on Wednesday joined 13 others in a lawsuit against the historic overhaul of the U.S. health care system signed into law last month by President Barack Obama, the Texas attorney general announced.
The lawsuit initially filed by Florida, South Carolina, Nebraska, Texas, Utah, Louisiana, Alabama, Michigan, Pennsylvania, Colorado, Idaho, South Dakota and Washington argues that the legislation's requirement that individuals buy health insurance violates the Constitution.
It calls the health care bill an "unprecedented encroachment on the sovereignty of the states" and asks a judge to block its enforcement.
"The Constitution nowhere authorizes the United States to mandate, either directly or under threat of penalty, that all citizens and legal residents have qualifying health care coverage," the lawsuit states.
White House Press Secretary Robert Gibbs said last month that the Obama administration expected to win any legal challenge to the health care bill.
Legal experts say the lawsuit is unlikely to succeed.
Renee Landers, a law professor at Suffolk University in Massachusetts, said the Constitution gives Congress broad power to regulate commerce and promote the general welfare of Americans.
"If the federal courts follow existing precedents of the United States Supreme Court, I don't think that the claims will be successful," Landers told CNN last month.
Separately, legislatures in three dozen states are considering proposed measures aimed at blocking elements of the health care bill. But Cal Jillson, a political science professor at Southern Methodist University in Texas, said the Constitution says laws passed by Congress trump state laws.