* Eleven states to join in collective lawsuit

* White House says legal challenges will not succeed

* Virginia: Congress cannot force people to buy insurance (Adds W.House reaction; Utah, S.Carolina, Tea Party efforts)

CHICAGO, March 22 (Reuters) - Republican attorneys general in at least 12 U.S. states warned on Monday that lawsuits will be filed to stop the federal government’s healthcare reform bill from encroaching on states’ sovereignty.

The lawsuits were announced hours after the U.S. House of Representatives gave final approval on Sunday night to a sweeping overhaul of the $2.5 trillion U.S. healthcare system, President Barack Obama’s top domestic priority.

The bill to be signed by Obama on Tuesday would extend medical coverage to 32 million uninsured Americans, expands the government health plan for the poor, imposes new taxes on the wealthy and bars insurance practices such as refusing to cover people with pre-existing medical conditions.

Eleven of the attorneys general plan to band together in a collective lawsuit on behalf of Alabama, Florida, Michigan, Nebraska, North Dakota, Pennsylvania, South Carolina, South Dakota, Texas, Utah and Washington. Factbox: [ID:nN22202437]

Utah Attorney General Mark Shurtleff told Reuters the lawsuit will probably be filed in U.S. District Court for the Northern District of Florida.

The case could be decided relatively quickly at the trial-court level since the question is mainly one of law, according to South Carolina Attorney General Henry McMaster, who said appeals and a U.S. Supreme Court ruling were likely in the lawsuit that seeks a declaratory judgment against the reforms.

“You won’t need to call expert witnesses or file a lot of briefs,” McMaster said. “This is a matter of law, not policy.”

White House spokesman Robert Gibbs said the Obama administration did not think the lawsuits will succeed.

“For many decades, the Supreme Court has recognized Congress’ authority under the commerce clause to regulate activities related to interstate commerce,” Gibbs said at the daily briefing.

CONGRESS VS STATES

State officials have cited potential commerce clause violations as well as the bill being at odds with the Tenth Amendment to the U.S. Constitution. That amendment says that “powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states.”

Virginia Attorney General Kenneth Cuccinelli, who plans to file a lawsuit in federal court in Richmond, Virginia, said Congress lacks authority under its constitutional power to regulate interstate commerce to force people to buy insurance. The bill also conflicts with a state law that says Virginians cannot be required to buy insurance, he added.

“If a person decides not to buy health insurance, that person by definition is not engaging in commerce,” Cuccinelli said in recorded comments. “If you are not engaging in commerce, how can the federal government regulate you?”

Forrest McDonald, a retired University of Alabama history professor who has written a book on states’ rights, said Congress has no power to make someone buy something.

“You can stretch it all to hell and you’re going to find a lot of power, but you can’t find the power to make me buy a car or anything,” he said.

But Mark Rosen, a Constitution scholar at Chicago-Kent College of Law, said the states do not really have a constitutional leg to stand on.

“Congress has clear authority to pass this type of legislation and under the supremacy clause that makes federal law supreme,” he said.

In addition to the pending lawsuits, bills and resolutions have been introduced in at least 36 state legislatures seeking to limit or oppose various aspects of the reform plan through laws or state constitutional amendments, according to the National Conference of State Legislatures.

So far, only three states -- Idaho, Virginia and Utah -- have enacted laws, while an Arizona constitutional amendment is seeking voter approval on the November ballot. But the actual enactment of the bill by President Barack Obama could spur more movement on the measures by state lawmakers.

No anti-healthcare reform legislation has emerged in Democrat-dominated states like Illinois and New York.

Meanwhile, Tea Party groups and others in Ohio and Michigan are readying proposed constitutional amendments to fend off the federal healthcare mandates after legislative attempts stalled. They hope to collect enough signatures to place the measures on the November ballot in those states.

Florida Attorney General Bill McCollum, a Republican candidate running for governor, said the mandate would cost Florida at least $1.6 billion in Medicaid alone.

All states would receive extra funding to cover Medicaid costs, which are expected to rise under the reform, including 100 percent federal coverage for new enrollees under the plan through 2016.

Medicaid is the healthcare program for the poor jointly administered by the states and federal government.