WASHINGTON (Reuters) - Twenty-one U.S. states led by Texas and Arkansas on Thursday announced a lawsuit against Delaware, accusing it of violating federal law by snatching up to $400 million in unclaimed checks that should have gone to other states.

"Delaware has our money," Texas Attorney General Ken Paxton said at a news conference outside the U.S. Supreme Court.

The suit was being filed directly at the Supreme Court, which typically hears cases only on appeal from lower courts but accepts certain lawsuits brought by one state against another.

Paxton, a Republican, said the amount owed to the 21 states bringing the case exceeds $150 million and could reach $400 million in unclaimed checks for 49 states.

The lawsuit focuses on "official checks" such as cashier's checks, traveler's checks and money orders, not personal checks.

The 21 states contend that Delaware and global financial transaction firm MoneyGram International Inc <MGI.O> violated federal law by sending uncashed MoneyGram checks to Delaware instead of returning them to where they were purchased.

Paxton said under federal law, the unclaimed checks should go to the state where the financial transaction was initiated. That means if an unclaimed official check purchased in Texas remained unclaimed, the proceeds should go to Texas.

But Paxton said Delaware has been playing by a different set of rules under which the proceeds of unclaimed checks from financial institutions incorporated in Delaware, where many businesses are incorporated, were sent to Delaware no matter where the transaction was initiated.

"This practice not only violated federal law, but it is wrong and it is unfair," Paxton said.

Thomas Cook, Delaware's secretary of finance, said the issue has been the subject of two prior lawsuits filed by Pennsylvania and Wisconsin in federal district courts this year.

"Delaware disputes the allegations in those suits and, to bring some clarity to this issue, filed an action in the U.S. Supreme Court last week to resolve the outstanding legal question," Cook said, adding that the state was puzzled why Texas did not intervene in the existing cases.

For Delaware, unclaimed money and property is a source of cash to plug budget holes, and account for 13 percent of the small state's annual revenue.

Dallas-based MoneyGram, which is incorporated in Delaware, said in a statement, "This is a dispute among states over which state has the priority to property escheated to the State of Delaware in good faith by MoneyGram."

The states that joined Texas and Arkansas in the lawsuit included: Alabama, Arizona, Colorado, Florida, Idaho, Indiana, Kansas, Kentucky, Louisiana, Michigan, Montana, Nebraska, Nevada, North Dakota, Ohio, Oklahoma, South Carolina, Utah and West Virginia.

(Reporting by Doina Chiacu in Washington, Jon Herskovitz in Austin, Texas, and Tom Hals in Wilmington, Delaware; Writing by Jon Herskovitz; Editing by Will Dunham)