Republicans' big victories in Congress could compel a second look at parts of President Barack Obama's sweeping healthcare overhaul, but any changes will be subtle and a far cry from the blanket repeal vowed by party leaders.

Democrats' continued control of the Senate, albeit with a smaller majority, and Obama's presidential veto power leave the Republicans with only a weak hand in trying to push through aggressive changes.

"Regardless of Republican rhetoric ... last night's Congressional results won't yield changes in healthcare reform law," said Leerink Swann analyst John Sullivan.

Senate Majority Leader Harry Reid opened the door to possible "tweaks" of the healthcare overhaul -- the president's signature legislative achievement.

For now, health insurers, whose shares staged a modest rise on Wednesday, must adhere to the law passed earlier this year.

As results rolled in on election night, the expected new House Majority leader, Representative Eric Cantor, vowed to repeal the bill. The healthcare reform, which was passed in both the House and the Senate without a single Republican vote, imposes tough new standards on health insurance companies and requires all Americans to buy coverage starting in 2014.