In a special House election Democrat Kathy Hochul won what had been a Republican seat in upstate New York, lifting Democrats' hopes for the 2012 campaign.
The win also serves a significant blow to the Republican efforts to reform Medicare.
Hochul upset Republican state Assemblywoman Jane Corwin in the special election in the conservative-leaning 26th district after tying Corwin to the controversial GOP budget plan that included a provision to turn Medicare into a voucher program.
As her campaign's centerpiece, Hochul attacked changes in the Medicare program proposed by House Budget Committee chairman Paul Ryan.
I will fight any plan that tries to decimate Medicare - that is something people in this district feel passionately about and I do as well, Hochul said in a debate last week with Republican opponent Jane Corwin, who supported the Ryan budget plan.
The win sends alarm bells ringing throughout the already Republican majority in the House of Representatives.
The plan to transform Medicare into a private voucher system within 10 years is still being endorsed by Republican leaders .
But the loss of the House election and a high-profile defection by an influential Senate Republican, Scott Brown of Massachusetts, on Monday has made it clear that the proposal has created a big political liability for the party.
The Republicans have said the plan was needed to reduce the soaring US budget deficit, but polls have shown it to be unpopular with voters.
With about three quarters of the precincts reporting, Hochul had 48 percent and Corwin 42 percent.