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Outside groups spent $3.8 million in Boston mayoral race

The two candidates who faced off in the Boston mayoral contest made history by spending a combined $5.6 million, a record amount for a Massachusetts municipal contest. But that was only part of the story — because outside groups poured another $3.8 million into the race, a new report says.

Both mayor-elect Martin J. Walsh and City Councilor John R. Connolly spent more money than any other mayoral candidate in state history, spending $2.7 million and $2.8 million, respectively, according to the report by the state’s Office of Campaign and Political Finance.

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But Walsh received the lion’s share of the outside spending, $2.5 million of the $3.8 million.

That spending became a major focus of the race as labor-affiliated groups bouyed the candidacy of Walsh, a former union president.

Special interest groups have two ways of spending money in elections: They can give limited donations directly to the campaign, and they can also make “independent expenditures” — money spent on behalf of the candidate but not handled by the campaign. Outside groups are legally barred from coordinating with a candidate’s campaign.

Political action committees spending money in the mayoral race are required to report expenditures within 24 hours in the final days before an election. But the state law allows donors to remain confidential until January. The state’s top campaign finance officials and several media outlets asked the groups to voluntarily release names before the election, but they declined.

At one point, Connolly called on both campaigns to swear off all outside money — but gave up after Walsh refused, and began allowing outside money to be spent on his behalf.

The total spending by the Walsh and Connolly campaigns topped the combined total spent by current-Mayor Thomas M. Menino and his 2009 opponent Michael Flaherty, who spent roughly $4 million combined.

Overall, the 12 candidates who participated in both the preliminary and final election spent $10.5 million.

Wesley Lowery can be reached at wesley.lowery@globe.com. Follow him on Twitter @WesleyLowery.
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