Republican state Sen. Scott Brown said Thursday that he had raised more than $600,000 in December for his long-shot Massachusetts Senate campaign.That more than doubles what Brown has reported in receipts through Nov. 18. But it is well behind the fundraising pace of state Attorney General Martha Coakley (D), the favorite in the race, who raised $4.2 million through Nov. 18. Coakley has not released her end-of-year fundraising totals yet.The Brown campaign also released plans Thursday for a “money bomb— scheduled for Jan. 11, eight days before the Jan. 19 special election. The fundraising technique, pioneered by the presidential campaign of Rep. Ron Paul (R-Texas), encourages donors to coordinate their giving on a certain day. Brown did not disclose his cash-on-hand total as of the end of the month but said the campaign would have “plenty of cash— for the final weeks of the race and was keeping to the budget it had planned out in advance.The campaign launched its first television ad of the campaign on Wednesday; previously, it had relied on Web ads and earned media to get its message out.Coakley comfortably won a four-way primary on Dec. 8 and is the heavy favorite in the general election contest to fill the seat held by the late Sen. Edward Kennedy (D). Brown did not face a competitive primary.