In the newest signal yet that Democrats are nervous about the Massachusetts Senate special election, the Democratic Senatorial Campaign Committee has made a $567,000 ad buy to boost state Attorney General Martha Coakley. The buy is the first independent expenditure by either national party in the race.Republicans see a growing opportunity to pull off an upset in next Tuesday’s special election to fill the seat of the late Sen. Edward Kennedy (D), and Democrats are working in overdrive to stave off disaster.Spending among conservatives is also ramping up in the race. The National Republican Trust political action committee will launch an ad campaign on behalf of state Sen. Scott Brown (R), joining two other conservative groups conducting independent campaigns either supporting Brown or attacking Coakley.Executive Director Scott Wheeler said in an interview that the PAC, which made about $100,000 in independent expenditures supporting Conservative Party candidate Doug Hoffman in a fiery upstate New York special election in the fall, is planning to make a statewide ad buy to begin airing either Friday or Saturday through Monday, the day before the Jan. 19 election. Wheeler said he had been keeping an eye on the race and decided to jump in after a Public Policy Polling survey released over the weekend showed the race in a dead heat. Other recent polls, however, have found Coakley leading by anywhere from single digits to 15 points. That is still far closer than expected given Coakley’s status as the heavy favorite going in and the Democratic registration advantage in Massachusetts.Wheeler said his PAC’s ad will focus on health care, which has become a flash point in the race given that Brown could become the pivotal 41st Republican Senate vote against the health care bill.The conservative American Future Fund has already spent $400,000 on an ad buy going after Coakley, and Our Country Deserves Better PAC also plans to make an independent ad buy on behalf of Brown in the days leading up to the election.