Introduction

U.S. Senate candidates, Republican Gabriel Gomez, left, and Democratic U.S. Rep. Edward Markey, right before a debate at the WGBH studios in Boston. (AP Photo/WGBH, Meredith Nierman)

Political allies of Democratic U.S Rep. Ed Markey and Republican Gabriel Gomez have together reported spending more than $5 million on advertisements designed to boost their preferred candidate in the Massachusetts U.S. Senate general election, according to a Center for Public Integrity review of federal records.

And Markey’s supporters have thumped Gomez’s backers: Since the contest began on May 1, pro-Markey super PACs, nonprofits, party committees and other political groups have accounted for 70 percent of the spending.

In the weeks after the race’s primary elections concluded, attacks by groups such as the Democratic Senatorial Campaign Committee and the Senate Majority PAC went unanswered.

During the GOP primary, Gomez received super PAC support from an outfit called the Committee for a Better Massachusetts, as the Center for Public Integrity previously reported. But the group has been largely dormant since he won the nomination.

Only since mid-June has a pro-Gomez outside cavalry arrived on the scene.

From June 13 through Sunday, Gomez-supporting groups have reporting spending more than $1.5 million on independent expenditures, essentially matching the expenditures of Markey’s outside allies in the final stretch of the campaign, according to the Center’s analysis.

The GOP Senate hopeful has been bolstered by the Massachusetts Republican Party, as well as two super PACs — Americans for Progressive Action and the American Unity PAC — although most polls show him far behind Markey.

Because of their late entry into the race, the donors fueling these two super PACs won’t be officially reported until after all the votes have been cast in Tuesday’s special election.

Billionaire hedge fund manager Paul Singer has been the driving financial force behind the American Unity PAC, which backs Republicans who support marriage for same-sex couples. Records show he gave the super PAC more than $1.7 million in 2012.

And on Friday, the Wall Street Journal reported that wealthy California vintner John Jordan, a longtime Republican donor, was providing the cash to fund pro-Gomez Americans for Progressive Action, which registered with the Federal Election Commission on June 6.

“I saw an American hero running in a close race in a tough state while getting absolutely pounded by Democrats throwing everything they could at him,” Jordan told the Journal. “I just couldn’t sit by and watch and leave him alone while the establishment Republican groups decided to sit on their hands and just leave him on the beach.”

The disclosure lapse is also true for the Senate Majority PAC, which has reported about $1.4 million in expenditures but did not start spending in the race until June 7.

Meanwhile, the NextGen super PAC, which has spent more than $500,000 on pro-Markey yard signs, aerial banners and online ads since May 1, counts billionaire Thomas Steyer as its main funder. Steyer has pumped at least $750,000 into the super PAC, records show.

Markey — a 20-term veteran of the U.S. House of Representatives who chaired the Select Committee on Energy Independence and Global Warming for years — has also been backed by a variety of labor unions and environmental groups, including the League of Conservation Voters.

Markey’s own campaign, too, has enjoyed a large fundraising advantage over Gomez. Markey raised more than $8 million for use during both the primary and general election — roughly double Gomez’s haul.

Notably, in the final stretch of the general election, Markey has received campaign donations from the likes of super donor trial lawyer Amber Mostyn, former Secretary of State Madeleine Albright, former Senate Majority Leader Tom Daschle and Harold Ickes, the longtime Democratic operative who headed the main super PAC supporting President Barack Obama’s re-election bid.

Gomez, a political newcomer, is a former Navy SEAL who resigned from a position at the Boston-based investment firm Advent International to pursue his U.S. Senate bid.

Adam Wollner contributed to this report.