Republican Scott Brown upset his Democratic rival Tuesday in liberal Massachusetts' special Senate election,jeopardizing President Barack Obama's health care overhaul as his first year in office winds down.

With 93 percent of the vote counted, Brown, who will become the 41st Republican in the 100-member Senate, defeated state Attorney General Martha Coakley by a margin of 52 percent to 47 percent.

The 50-year-old Brown will fill the seat of the late Democratic Senator Edward M. Kennedy, a top advocate of health care reform who died in August after nearly 47 years in office.

Big blow to Democrats

Brown's victory in Massachusetts, a traditionally Democratic-leaning state, dealt a stout blow to Obama and Congressional Democrats, jeopardizing their efforts to reform the health care system.

Brown, a state senator who defined himself during the campaign as a truck-driving everyman who came from humble beginnings, has promised to cast the 41st vote that Republicans need to filibuster -- a procedure used to stall and defeat legislation -- final passage of the health care measure.

The House and Senate passed their own health care bills last year and are now negotiating a reconciled version to be sent back to both chambers for final approval.

However, with Brown's victory, Democrats now lose the 60-vote Senate "super majority" that enabled them to pass a bill without any Republican support.

Moreover, the victory, together with Republicans' seizure of Virginia and New Jersey, both Democratic strongholds, in November's gubernatorial races, reflect big political problems for the president's party in the mid-term Congressional and gubernatorial elections this fall.

Why did Democrats lose?

Democrats outnumber Republicans in Massachusetts by a 3-1 ratio, according to state registration figures.

However, more than half of Massachusetts voters' -- or 2.1 million people -- are independents not affiliated with any political party.

In other words, it was up to the independents to decide the winner in the Senate race.

Coakley just weeks ago led Brown, who became the first Republican senator from Massachusetts in 30 years with his victory, by a comfortable margin but recent surveys found that her lead had been totally erased.

In a recent Suffolk University poll, for instance, Brown led by 4 percentage points. Among independents, Brown led 65 percent to 30 percent

Why did independent voters, who overwhelmingly voted for Obama in the 2008 presidential election, turn to the Republicans?

The shift reflects growing anti-Washington sentiments among voters, analysts said. The fact, too, is that people are dissatisfied with the sluggish economy, high unemployment, Wall Street bailouts, the huge deficits and partisan wrangling over health care.

A cornerstone of Brown's campaign was his promise to vote against the health care plan.

Healthcare reform in danger

House Speaker Nancy Pelosi, a Democrat from California, said earlier Tuesday that Congress would soon pass the health care overhaul bill, no matter who replaces Kennedy in Massachusetts.

Despite Pelosi's reassurances, many analysts now doubt that the Democrats' year-long effort to advance health care reform is a given.

Nevertheless, Democrats still have options to salvage the bill, they said.

One unlikely way is to act quickly to reach a compromise deal before Brown is sworn in.

Massachusetts law permits 15 days for delivery of final election results to the secretary of state, who must certify the vote before a new Senator can be sworn in.

Another way is to require that House Democrats swallow hard and approve the Senate bill without any changes, and Obama could sign it into law without another Senate vote.

Both options, however, seem extremely difficult to take, given the huge differences on health care between the House and Senate.

A third option is to seek the support of a Republican senator and let him cast the crucial 60th vote needed to pass a reconciled version of the healthcare legislation.

Some Democrats hope Senator Olympia Snowe, a Republican from Maine, might become that vote but others seriously doubt it.