Outside super PAC spending in the Massachusetts governor’s race has nearly doubled the outside spending totals set four years ago, and the wave of outside money has been decidedly one-sided.

Most of the increase in outside spending has come from a single source, the Republican Governors Association. The RGA has poured $12.4 million into the race between Charlie Baker and Martha Coakley, and in doing so, the national Republican group has outspent the gubernatorial field. The two major candidates for governor, their running mates, and their political parties have combined to spend less this year than the RGA has spent.

Super PACs, labor groups, and wealthy individuals may legally raise and spend unlimited sums of money on political races, as long as the outside groups do not coordinate political strategy with the candidates they support. The 2010 Massachusetts governor’s contest attracted roughly $11 million in outside spending from labor unions, and from national Democratic and Republican groups.

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Outside spending in this year’s gubernatorial race has far outpaced the $11 million mark from 2010. Super PAC spending in the contest between Baker and Coakley has already exceeded $20 million, and that total may edge higher, as outside groups report last-minute expenditures.

The Republican Governors Association has more than doubled its spending pace from four years ago. The group is responsible for four of every five dollars in super PAC spending above levels from 2010. The RGA’s $12.4 million in spending has come through a pair of super PACs, with $11.8 million going to television, and the rest to direct mail. The group’s spending marks an enormous step up from the $4.7 million it devoted to the 2010 contest between Baker and Gov. Deval Patrick. It has outspent its partisan rival, the Democratic Governors Association, by a margin of nearly 9-1.

The largest source of outside spending on the Coakley side has been the Massachusetts Teachers Association. The union has contributed more than $2.5 million to an anti-Baker super PAC.

Excluding money spent in support of state Treasurer Steve Grossman’s Democratic primary campaign, Republican-allied super PACs hold a $5 million spending edge over Democratic super PACs in the governor’s race. In 2010, Patrick’s campaign was outspent by Baker, but Democrats used an edge in super PAC spending to even the money contest. This time around, heavy Republican super PAC spending has compounded a cash shortage on the Democratic side.

Between direct campaign expenditures and super PAC spending, Coakley’s camp has been outgunned by $7.3 million. The RGA spent more money last week ($3.75 million) than Coakley’s campaign has reported spending since January 1 ($3 million).

The heavy RGA spending, combined with a slight uptick in super PAC spending on the Democratic side, means that a strong majority of the money being spent in the Coakley-Baker contest is coming from groups that have no formal connection to the campaigns themselves. All told, $6 of every $10 spent on the Coakley-Baker matchup has come from an outside PAC.