A new radio ad released Tuesday by the National Republican Congressional Committee attacking Democratic 2nd District candidate Emily Cain over a proposed measure against childhood obesity highlights a surge in spending by outside groups in a key race that’s attracting national attention and is the most expensive U.S. House contest in Maine history.

Also on Tuesday, the Congressional Leadership Fund, a political action committee endorsed by House Republican leadership, announced a $600,000 campaign against Cain that will launch Oct. 25 with advertisements across Maine.

The influx of outside money and advertising is the latest sign that Maine’s 2nd District race – featuring Cain against incumbent Republican U.S. Rep. Bruce Poliquin – is considered an important toss-up as the national political parties battle for control of Congress in the November election. All told, more than $2 million has been spent or pledged so far by outside national groups in Maine’s 2nd District race.

“We’ve been watching this race closely,” said Ruth Guerra, deputy communications director for the Congressional Leadership Fund, adding that the group already has spent $21 million across 16 congressional races in the U.S. “It wouldn’t be rare for us to increase our digital or increase our TV as time goes by in any of those races.”

The NRCC ad released Tuesday plays through a scenario of a teacher asking students to line up so they can have their weights recorded. It’s a criticism of L.D. 1886, a bill that Cain introduced in the Maine Legislature in 2007 aimed at gathering data on childhood obesity.

The ad leaves out the fact that height and weight reporting would be confidential and that parents would have the option to opt out if they chose to. It calls the proposal, which never made it out of a legislative committee, a “massive violation of kids’ privacy” and warns that more government bureaucrats would have had to collect the data.

The ad, sharply criticized by a Cain spokesman on Tuesday, points to growing outside spending in the 2nd District, where the National Republican Congressional Committee so far has spent more than $500,000 on advertising and the Democratic Congressional Campaign Committee has spent close to $300,000, according to Federal Election Commission filings.

A recent Portland Press Herald/Maine Sunday Telegram poll, conducted by the University of New Hampshire Survey Center, shows Poliquin with a 10 percentage point lead over Cain, whom he defeated in a three-way race in 2014 by 5 percentage points. That race was the most expensive House contest in Maine history, with the candidates bringing in a combined $3.7 million, a figure they had already exceeded by the end of June this year, when Cain had raised more than $1.6 million and Poliquin about $2.6 million.

“If something sounds too crazy to be true, it’s probably a lie,” said Dan Gleick, communications director for the Cain campaign, in a statement Tuesday reacting to the new radio ad. “This absurd attack is Bruce Poliquin’s reward for siding with Wall Street at the expense of Maine’s middle class. Nonsense funded by secret donors doesn’t change Emily’s incredible record of fighting for working families and children.”

Outside groups also have gone after Poliquin, who took issue with an ad released last month by the group End Citizens United, a Democratic political action committee, criticizing Poliquin for voting in support of a Medicare voucher system and calling him a “Wall Street banker.”

“This attack ad is nothing more than a political trick and their deceitful ad should be removed immediately,” Poliquin’s press secretary, Michael Byerly, said in a news release at the time. “Congressman Bruce Poliquin has always voted in the best interest of Maine seniors, like his parents, by supporting legislation that will strengthen and secure Medicare and Social Security.”

End Citizen’s United has contributed more than $200,000 to ads opposing Poliquin to date, according to the FEC.

The House Majority PAC, which is focused on helping Democrats win re-election, has spent a similar amount in support of Cain.

The NRCC has $2.35 million reserved in ad spots in the 2nd District, while the DCCC announced earlier this year a total of more than $800,000 in ad reservations in Bangor and Portland.

On Tuesday, spokesmen for both committees said the race is a priority for them, though they declined to say what future spending might look like.

“Given Congressman Bruce Poliquin’s record of rigging the system for his own benefit, it’s no surprise that he is one of the most vulnerable House incumbents in the country,” said Bryan Lesswing, Northeast regional press secretary for the DCCC. “Whether it’s abusing loopholes to escape his taxes or repeatedly failing to pay his own property taxes on time, Congressman Poliquin has always been out for himself, and voters will reject him this November.”

NRCC spokesman Chris Pack fired back that Cain “is far too extreme for Maine families and it’s important that she doesn’t get a chance to bring her extreme agenda to Washington.”

Meanwhile, the candidates are expected to debate Oct. 19 and Oct. 26 in forums held by Presque Isle-based WAGM-TV and Portland-based WCSH-TV.

In a statement Tuesday, Cain criticized Poliquin for turning down invitations from three additional stations for the opportunity to debate.

“Bruce Poliquin has a long record of not being straight with the people of Maine, and he clearly doesn’t want to answer questions about his record of selling out working families to Wall Street in exchange for campaign cash,” Cain said. “This is just the latest case of Bruce Poliquin doing what is best for Bruce Poliquin without a thought to what is best for regular people.”

Poliquin, in his own news release, said the two scheduled debates “will provide coverage to Mainers from Androscoggin and Oxford County, to Penobscot, Aroostook, Hancock County and everything in between.”

“I am looking forward to the upcoming debates and setting the record straight,” he said.

Byerly, Poliquin’s press secretary, did not respond to a call seeking additional comment late Tuesday.