**Voters are preparing to go to the polls in Massachusetts in a special election to fill the US Senate seat left vacant by Edward Kennedy’s death last August.**The race, between Democratic Attorney General Martha Coakley and conservative Republican state senator Scott Brown, is unexpectedly close.
If it goes to Mr Brown, the Democrats will lose their filibuster-proof 60-seat majority in the 100-seat Senate.
This would make it harder for Barack Obama to pass a healthcare reform bill.
Healthcare reform is the most important domestic policy objective of his first year as president.
The BBC’s Paul Adams, in Boston, says this Senate race should have been a formality for the Democrats - they outnumber Republicans in Massachusetts by around three to one, and the seat being contested was controlled by the Kennedy family for more than half a century.
But he adds that Ms Coakley has run a lacklustre campaign, allowing her Republican opponent - with vigorous support from conservative activists - to overtake her in the polls.
Mr Obama went to Boston on Sunday to lend support to the Coakley campaign, a sign of the growing apprehension felt by the White House.
The Democrats believe that their relentless portrayal of Mr Brown as a friend of Wall Street is working, but with heavy snow blanketing much of the state, motivating supporters to show up and vote will be critical.
Analysts say that with opinion polls showing nearly half of all Americans think President Obama is not delivering on his major campaign promises, the Massachusetts race could be seen as a referendum on his first year in office.