(Reuters) — Democrats were prepared to quickly come up with a legislative solution should the U.S. Supreme Court rule in the next few days to invalidate a central part of President Barack Obama's signature health care law, party leaders in the Senate said Tuesday.

The high court is expected to rule by the end of June in a case that challenges tax subsidies that are helping millions of Americans afford health insurance premiums under the 2010 Affordable Care Act.

The plaintiffs in the case argue that the law's language restricts these subsidies to states that have established their own health insurance exchanges under the law.

A ruling in the plaintiffs' favor could mean that 6.4 million low- and middle-income Americans in the 34 states that use the federal health insurance exchange will lose their tax subsidies and possibly their insurance.

"The Supreme Court, I'm hopeful and confident, will rule the right way. If they don't, we're ready to move on it quickly," Senate Minority Leader Harry Reid, D-Nev., told reporters.

Sen. Reid did not elaborate. But Dick Durbin, D-Ill., the No. 2 Senate Democrat, said that if the administration loses the case, Democrats would offer a short piece of legislation clearly saying the tax subsidies are also available to people on the federal exchange.

"It's one sentence, and it's already been written," Sen. Durbin said in a Capitol hallway. "I hope we don't need it," he added.

Democrats generally have been reluctant to talk about the possibility that the administration might lose the case.

"Leader Pelosi is confident that the court will not act to kick more than six million Americans off their health insurance," Drew Hamill, spokesman for House Minority Leader Nancy Pelosi, D-Calif., said Tuesday.

Leading Republicans, who have opposed the health care reform law from its inception, say they will have a legislative plan of action if the court throws out the subsidies. But they have not provided many specifics, saying they will do so after the court rules.

In both the House and Senate, some Republicans say they would favor extending the subsidies for up to two years while working on other changes to the law. Those changes could include letting states opt out of the ACA to set up their own insurance systems.