Independent spending is ramping up in the Massachusetts Senate special election, as Republicans see a growing opportunity to pull-off an upset and Democrats seek to stave off disaster.

In fresh signal of Democratic nervousness about the race, the Democratic Senatorial Campaign Committee has made a $567,000 ad buy in the race, in Boston and Springfield markets, the first independent expenditure by either national party in the race.

Conservative political action committee the National Republican Trust PAC, meanwhile, will launch an ad campaign in behalf of Massachusetts Senate candidate Scott Brown (R), joining two other conservative groups conducting independent campaigns either supporting Brown or attacking his opponent, Massachusetts Attorney General Martha Coakley (D).