Outside PACs and unions are funneling money into Boston at a rate that far outpaces anything seen nationally in other recent big-city mayoral races. The mayor’s contest is awash with semi-regulated campaign funds – money that doesn’t carry the fundraising limits and disclosure requirements political candidates have to comply with.

Super PACs and labor unions have upended Boston’s mayoral race, turning what was a relatively even spending contest between Rep. Marty Walsh and City Councilor John Connolly into a tremendously lopsided affair: An unprecedented amount of outside money has saturated Boston’s mayoral race, and more than $4 out of every $5 is lining up behind Walsh.

Boston, supposedly the Athens of America, has become nation’s dark money capital.

Direct campaign expenditures by corporations, unions, and wealthy individuals have risen dramatically since the Supreme Court’s 2010 Citizens United v. FEC decision. Citizens United eliminated fundraising and spending limits on outside political committees. So long as there’s no coordination with political candidates, outside groups can spend unlimited amounts of money in federal, state, and local races. The source of these groups’ funding can often be opaque; while some file disclosure reports with campaign finance regulators, others accept funds funneled through nonprofits and shell corporations that are exempt from disclosure requirements.

To date, super PACs and labor unions have poured $2 million into Boston’s mayoral race. The vast majority of that money — $1.65 million — has been spent on Walsh’s behalf. By contrast, outside groups have spent less than $190,000 promoting Connolly’s candidacy. The pro-Walsh tally rises daily.

These outside expenditures represent an enormous influx of cash into a relatively low-budget race. As of mid-October, Boston’s dozen candidates for mayor had spent a combined $7.9 million. Outside groups have already boosted this record spending by another 20 percent. In a state where campaign finance laws limit candidates to accepting contributions in $500 increments, super PACs and outside unions are spending money at a rate few political candidates could match.

The bulk of outside expenditures in the mayor’s race have been spent promoting Walsh, significantly tipping the balance of spending in the race. To date, Walsh’s campaign committee has outspent Connolly’s by roughly $190,000. But once pro-Walsh PAC and union funds are factored in, Connolly is being outspent in the race by $1.65 million, or roughly two-to-one.

The pro-Walsh outside groups have outspent 10 of Boston’s 12 mayoral candidates, even though the race for mayor began in earnest in April, and the pro-Walsh groups didn’t begin spending money until August. The groups aligned with Walsh have spent nearly the combined total spent by mayoral candidates Felix Arroyo, John Barros, Rob Consalvo, and Charlotte Golar Richie.

Since August 1, pro-Walsh unions and super PACs have outspent every candidate in the field.

The level of outside spending, relative to Boston’s size, dwarfs super PAC and union spending in other recent big-city mayoral contests. Los Angeles’s recent mayoral race attracted $9.9 million in outside spending, while New York’s ongoing mayoral contest has generated $5.8 million in outside spending to date. On a per-capita basis, however, super PACs and unions have poured four times more money into Boston than they’ve put into New York. Per-capita super PAC spending in Boston so far is 13 percent greater than levels seen in the completed contest in Los Angeles, where labor unions poured millions into Wendy Greuel’s unsuccessful candidacy.

Seattle, a city roughly the same size as Boston, has seen $365,000 in outside money spent in its current mayoral race – less than one-fifth Boston’s total. When Rahm Emanuel took the Chicago mayor’s office in 2011, he did it without any outside funds spent on his behalf, and with just $8,700 spent opposing him.

A pair of labor-friendly super PACs, American Working Families and Working America, have paced the pro-Walsh spending, although the campaign has also benefitted from outside spending from local and national unions representing service employees, hotel workers, and public employees. As super PACs, both American Working Families and Working America can raise and spend unlimited sums of money.

American Working Families, a Virginia-based committee run by Democratic media consultant Bud Jackson, has spent more than $900,000 producing and airing pro-Walsh advertisements.

Working America, a super PAC affiliated with the national AFL-CIO, has poured $557,000 into the race. Working America has largely concentrated on running a paid canvassing effort on Walsh’s behalf, but the PAC has also spent more than $114,000 on printing and mailing. The group is responsible for a recent mailer that cast Connolly as a corporate lawyer and “son of privilege.” Walsh criticized the negative mailing.

Outside spending became a flashpoint earlier in the race, when the education reform group Stand for Children announced plans to spend at least $500,000 on Connolly’s behalf. Connolly quickly renounced the planned expenditure, and he asked that the outside group Democrats for Education Reform pull pro-Connolly canvassers off the streets. Since the Stand for Children controversy, Connolly has repeatedly asked, both before and after September’s preliminary election, that all mayoral candidates swear off outside expenditures.

Walsh, the recipient of the bulk of these outside expenditures, has refused. Instead, he’s said he doesn’t trust Connolly to stick to a pledge against outside money, and has raised the specter of Stand for Children jumping into the race with as much as $3 million. Connolly said Walsh had pulled that claim from thin air, while Stand for Children called it fabricated. Despite Connolly’s opposition, Democrats for Education Reform announced plans this week to spend $115,000 on a pro-Connolly television ad.