The court is expected to make a decision in June on the health care legislation, which President Obama signed into law in March 2010 after months of public and Congressional debate. Some parts of the law are already in effect, but major provisions including the mandate requiring most people to buy health insurance don't kick in until 2014.
The mandate is arguably the most controversial part of the law, and for health insurers, it would mean a big increase in both customers and expenses.
While it's difficult to predict what the court will do based on oral arguments, analysts Dave Shove of BMO Capital Markets and Sarah James of Wedbush both said that the Medicaid expansion part of the law is probably safe. More than half of the 30 million people who would gain health insurance coverage under the new law would be covered through Medicaid.
James said she spoke to a panel of three legal experts, and all the panelists think the court will uphold the Medicaid provision.
"It is almost inconceivable the court would find the Medicaid expansion unconstitutional since a law has never been found unconstitutional under the argument the states (who are opposed to the law) are using," she wrote, summarizing the panelists' views. James said investors are expecting the same outcome.
Shove wrote that that the Medicaid expansion is similar to other changes made since Medicaid was created in 1965.
Shove said health insurance stocks should do well until the ruling comes down because business trends have been strong and should surpass Wall Street expectations. Shove rates shares of UnitedHealth Group Inc., WellPoint Inc. and Aetna Inc. "Outperform," and he has "Market Perform" ratings on seven other companies in the industry.
UnitedHealth is the largest health insurer in terms of revenue, and WellPoint is the largest by enrollment. Shares of UnitedHealth rose $2.67, or 4.8 percent, to $58.11 Thursday and WellPoint's stock picked up $1.62, or 2.3 percent, to $71.62. Aetna shares gained $3.04, or 6.5 percent, to $49.56.
Shove said he expects the law to be altered in Congress in 2013 no matter what the court decides, but he expects the individual mandate will be stricken from the law.
"We expect the Justices to hold that the mandate is unconstitutional and sever it from the law, forcing Congress to rework this legislative Rube Goldberg machine," Shove wrote in a note to clients.
Also trading higher were shares of Humana Inc., which added $2.90, or 3.3 percent, to $91.54, and Cigna Corp., which advanced $1.90, or 4 percent, to $48.97.
James said it will be good for Medicaid-focused insurers like Centene Corp., Molina Healthcare Inc., Amerigroup Corp., and WellCare Health Plans Inc. if the Medicaid provision is upheld. However those stocks were little changed on Thursday: Centene shares lost 28 cents to $47.45, Molina shares fell 29 cents to $33.44, Amerigroup shares declined $1.35 to $65.63, and WellCare stock rose 73 cents to $70.58.