AUGUSTA — Democrats and Republicans have already poured more than $43,000 into a special election later this month that both parties see as a portent to the 2014 legislative elections.

Voters in Senate District 19 head to the polls on Aug. 27 to elect a replacement for Seth Goodall of Richmond, a three-term incumbent Democrat who served as Senate majority leader before resigning last month to take a presidentially appointed post with the Small Business Administration. Vying for the seat are Republican Paula Benoit of Phippsburg, Democrat Eloise Vitelli of Arrowsic and Green Independent Daniel Longley Stromgren of Topsham.

All three are running as Maine Clean Election Act candidates, which means their campaign fundraising and spending are limited. But outside money, mostly directed by Maine Democratic and Republican state organizations, is flowing freely because the special election holds potential to both shrink the balance of power in the 35-person Maine Senate — where Democrats held a 19-15-1 advantage before Goodall’s departure — and serve as a preview of the 2014 legislative contests.

Democrats pointed to Somerville Democrat Chris Johnson’s victory in a February 2012 special election in nearby Senate District 20 as a momentum builder for the party’s surge to reclaim majorities in the House and Senate in the November 2012 general election.

Political groups began spending thousands of dollars on newspaper and radio advertisements, as well as mailers, late last month, according to financial disclosures filed with the Maine Ethics Commission. The independent expenditures don’t include spending by the campaigns themselves and by law, the groups are not allowed to coordinate their efforts with the candidates’.

As of Monday afternoon, the Ethics Commission reported that Republican-friendly sources have outspent Democrats by about $5,000. Spending on behalf of Benoit, who served one term in the Senate before being unseated by Goodall in 2008, totalled $24,328. Spending on behalf of Vitelli stood at $19,319 as of Monday afternoon. There were no expenditures reported on behalf of Stromgren.

The expenditures were as follows:

• The Senate Democratic Campaign Committee reported spending $833 on July 26 for advertisements in The Cryer, a weekly newspaper based in Brunswick.

• The Maine Republican Party reported on July 27 that it had spent $16,173 on web design ($10,000) and direct mailers ($6,173) through a firm called Rockwood Solutions Inc. in Manchester, N.H. On Aug. 1, the party reported hiring Rockwood Solutions for $550 worth of phone bank work, followed by another $7,378 contract with Rockwood on Aug. 8 for more mailers ($5,789) and phone banks ($1,588).

• The Maine Democratic State Committee reported spending $12,487 on Aug. 2 for radio advertising though a firm called CD2 Consulting in New Gloucester; and $6,000 for mailers on Aug. 7 through Ourso Beychok Inc. of Baton Rouge, La.

• A political action committee called Paving the Way to a Prosperous Maine, which is based in Winterport, home of Senate Republican Leader Michael Thibodeau, on Aug. 8 reported spending $400 for campaign literature with Express Graphics of Garland, Texas.

Lizzy Reinholt, spokeswoman for the Maine Democratic Party, said in addition to the expenditures, the Democrats have a core of volunteers who are going door to door and manning phone banks.

“This is a district that could go either way in any election year,” she said. “This race is happening in the middle of the summer at a time when most people aren’t thinking about going to a ballot box. Our resources are focused on making sure voters know who Eloise is and when Election Day is.”

The liberal-leaning Maine People’s Alliance circulated a fundraising email on Friday under the headline “LePage’s candidate is winning!” It cited data from the Maine secretary of state’s office that showed Republicans have far outpaced Democrats in requesting absentee ballots. It also tried to link Benoit to Gov. Paul LePage and said that a Benoit win could put a controversial expansion of Medicaid in Maine at risk.

“This election is critical,” read the email. “A single vote could make the difference on important issues like health care expansion.”

A mailer paid for by the Democratic Party also brought up health care and LePage, including the governor’s picture.

“In the Senate, Eloise will work with anyone to increase health care coverage and benefits for Maine people, including the expansion of Medicaid — something Governor LePage has opposed,” reads the mailer.

The effort to link Benoit to unpopular LePage positions and behaviors mirrors a strategy Democrats and their allies used in the 2012 elections. U.S. Rep. Mike Michaud, the Democrats’ likely challenger to LePage in 2014, is scheduled to join Vitelli for door-to-door campaigning Saturday, according to Reinholt.

Republicans have countered with campaign tactics designed to appeal to conservative and moderate voters by portraying Democrats and their supporters as liberals who are out of step with Maine values.

Maine Republican Party Chairman Rick Bennett emphasized that point Monday with an email blast in response to a similar email from the liberal Maine People’s Alliance over the weekend.

“This means only one thing — that swarms of liberal activists will be flooding the district, knocking on doors and making phone calls,” wrote Bennett. “They will do whatever it takes to win. … Every moment that we are not talking to voters is a moment that liberal Maine People’s Alliance activists will be saying negative things about Paula and trying to fool the voters in this election.”

Jason Savage, executive director of the Maine Republican Party, said he expects a close election between Benoit and Vitelli. He declined to provide copies of mailers or radio advertisements paid for by his party.

“There is no doubt we want to win it, but it’s really on the Democrats to try to defend it at this point,” said Savage. “There’s no doubt we want to win it.”

Senate District 19 includes the Sagadahoc County municipalities of Arrowsic, Bath, Bowdoin, Bowdoinham, Georgetown, Phippsburg, Richmond, Topsham, West Bath and Woolwich, and the town of Dresden in Lincoln County.