AUGUSTA — Six races for Maine Senate seats that could shift the balance of power at the State House in 2017 are attracting a growing volume of shadowy campaign spending from political action committees and the two major parties.

The seats at issue are held or sought by some of Maine’s highest-profile lawmakers, including Senate President Mike Thibodeau, a Republican from Winterport in Senate District 11, and Jeff McCabe of Skowhegan, the Democratic majority leader in the House who is running for the seat representing Senate District 3.

Jeff McCabe of Skowhegan, left, the Democratic Majority Leader in the House, and Senate President Mike Thibodeau, right, a Republican from Winterport. Related Headlines Database: Independent expenditures in the 2016 state legislative races

Spending by party committees and PACs, know as independent expenditures, had reached $492,000 for all legislative races by Oct. 4. That exceeds the $475,000 spent on legislative campaigns through September 2014.

About 92 percent of this year’s independent spending has targeted seats in the Senate, with $306,051, or 62 percent of the total, flowing toward six key districts, according to campaign finance reports filed with the Maine Commission on Governmental Ethics and Election Practices.

Much of the money is being spent to support candidates who have refused to accept private financing and are running taxpayer-funded campaigns under the Maine Clean Elections Act. It’s unlawful for any candidate to coordinate his or her campaign with the parties and PACs that are spending money on their races.

There is no limit on these independent expenditures – and a good portion of the money is being spent to attack the opponent of a preferred candidate.

“A lot of people are offended by the negative tone and the amount of resources that are being poured into some of these small state Senate races,” said Thibodeau, who won re-election to the Senate seat representing rural Waldo County in 2014 by only 100 votes over Democratic challenger Jonathan Fulford.

Republicans hold 20 of the 35 Senate seats and they are hoping to maintain their political edge, while PACs and party committees supporting mostly Democratic candidates, at this point, are spending at a pace rivaling some of most expensive election cycles in recent Maine history. That includes 2014, when a whopping $14 million in donations from across the U.S. was spent trying to influence elections for governor and the Legislature.

It’s spending that pays for everything from television and radio attack ads to post cards sent to voters touting a candidate’s virtues or accomplishments. Increasing amounts are also being channeled toward social media for advertising on Facebook, while still additional sums are being spent for boots on the ground door-knocking campaigns.

A new law, passed by a citizens initiative in 2015, now requires that the top three donors to each PAC be identified in the advertising the PAC buys. But in many cases the donations are coming largely from other PACs, which makes it difficult to identify the original source of the money, despite the public reports filed with the state ethics commission.

Democratic PACs and committees are targeting races that were either close in 2014 or have been left open by retiring lawmakers. They also are spending on efforts to unseat incumbent Republicans like Thibodeau or Sen. Rodney Whittemore, R-Skowhegan, who is facing a challenge from McCabe in District 3.

Thibodeau said he expects his opponent, Fulford, to gain support from Progressive Maine, a new PAC with $500,000 in contributions that is run from an office in Oakland, California. Still, Thibodeau shrugged off the notion that outside spending would hold much influence in his district. In 2014, $100,000 in such spending did not win Democrats that seat, Thibodeau noted.

“The seat in Waldo County wasn’t for sale last election cycle,” he said, “and I doubt very much it will be for sale this election cycle.”

Fulford is a Clean Elections candidate.

In Senate District 3, outside groups have spent $72,130. Over half of that, $37,822, was spent to support McCabe, and $25,689 was spent in opposition to Whittemore. Outside spending in support of Whittemore totaled just $7,045, and there was only $1,575 in opposition spending against McCabe. Overall, McCabe, the Democrat, had a more than 7-to-1 advantage in spending.

McCabe and Whittemore, both Clean Elections candidates, also have spent on their races. As of their latest campaign finance reports filed at the end of September, McCabe had an available war chest of $24,951 compared with Whittemore’s $23,160.

Jason Savage, executive director of the Maine Republican Party, said PACs supporting Democrats may be spending more money on their candidates, but it hasn’t always worked out well.

“It’s going to take a whole lot more money than that to convince voters that Jeff McCabe is the right candidate for that district,” Savage said. “This is a good example of Democrats making decisions out of Portland without any real idea of what it’s like in places like Somerset County and not really grasping what it is that people are concerned about or the mood of the electorate.”

Savage said he expects more spending in the last weeks of the campaign on opposition or contrast-type attack ads. He said neither party had a monopoly on the tactic, although Democrats seem to have more money for it.

McCabe doesn’t know how much outside groups have spent on the District 3 race, but he said he is aware of both positive and negative campaign material that has been given to voters.

“It doesn’t surprise me that I have a lot of support from folks, and the reality is I am a good fit for this district,” McCabe said. “If you see the positive pieces that are going out, they are highlighting my resume and my biography and are about where I come down as a person and things that I am passionate about.”

Whittemore, McCabe’s opponent, did not respond to several messages seeking comment.

Maine Democratic Party Chairman Phil Bartlett said the independent expenditures by the party’s committees reflect “a groundswell of support among hundreds of grassroots donors who are eager to see a Democratic Senate.”

Efforts for Fulford and against Thibodeau were a result of voters being “frustrated with Thibodeau’s failure to take action against Gov. LePage, who himself has predicted that his party will lose the Senate,” Bartlett said.

Under Maine’s campaign finance laws, groups that spend for or against a candidate must report expenditures within two calendar days until Oct. 25. After that, they must report expenditures within 24 hours until Election Day.

September spending, especially by independent groups, helps to identify races that the parties see as winnable.

Here’s a breakdown of the other races where independent expenditures are going to be a factor in November:

SENATE DISTRICT 2

This race features a contest for an open seat being vacated by Republican Michael Willette of Presque Isle. Running is Michael Carpenter, a Democrat and former Maine attorney general from Houlton, and Ricky Long, a Republican and outgoing member of the Maine House from Sherman.

At $55,423 in outside spending by Oct. 4, the race is shaping up to be the second most costly in 2016. Like his Democratic counterparts, Carpenter appears to be enjoying the upper hand in the form of outside help, with close to $35,000 being spent supporting him and another $20,000 being spent in opposition to Long.

Both are Clean Elections candidates. Long had about $12,000 on hand at the end of the month compared with Carpenter’s $10,900.

SENATE DISTRICT 16

This central Maine race sees incumbent Sen. Scott Cyrway, R-Benton, defending a seat he won from Democrats just two years ago against outgoing state Rep. Henry Beck, D-Waterville. So far, Beck has enjoyed just over $19,000 of support from independent expenditures compared with Cyrway’s $8,000. Meanwhile, PACs opposing Cyrway already have spent over $17,000. Both are Clean Elections candidates.

SENATE DISTRICT 18

This race in western Maine’s Oxford County features incumbent Sen. John Patrick, a Rumford Democrat and paper mill worker, facing Republican challenger Lisa Keim, a Dixfield Republican who has previously run unsuccessfully for the Maine House. Patrick’s supporters appear nervous about Keim this year, already spending $48,500 in support of Patrick. So far, Keim has seen no outside help in her race, but there also has been no independent spending to attack her. Both Keim and Patrick are also publicly financed Clean Elections candidates. Keim had about $16,500 in campaign cash on hand at the end of September compared with Patrick’s $23,800.

SENATE DISTRICT 1

This is another northern Maine race for an open seat, where former Sen. Troy Jackson, a Democrat from Allagash, is trying to recapture the seat he left to run for the U.S. Congress in 2014. He faces opponent Timothy Guerrette of Caribou. Guerrette became the Republican candidate after Republican incumbent Peter Edgecomb announced his retirement. The $35,761 of independent expenditures in the race are again lopsided in favor of the Democrat, with $31,761 being spent in support of Jackson. Another $4,000 has been spent to oppose Guerrette. Both Jackson and Guerrette are Clean Elections candidates. Jackson had $36,791 in cash on hand after the last reporting period compared with Guerrette’s $15,515.

Send questions/comments to the editors.