The top super PAC backing House Democrats is dropping more than $1 million into a last-minute media blitz defending the chairwoman of the Democratic Congressional Campaign Committee (DCCC) in a sign that her campaign for a new term is not guaranteed.

The House Majority PAC has reported spending almost $1 million on airtime and another $30,000 on digital advertising on behalf of Rep. Cheri Bustos (D-Ill.), who heads the DCCC, according to filings made with the Federal Election Commission (FEC).

Democratic sources in Washington and Illinois said two Republican polls released earlier this month that showed Bustos only narrowly ahead of her Republican rival, attorney Esther Joy King, were catalysts for the late spending blitz.

Bustos spent the last several weeks working the phones to raise money for her own campaign from both donors in Illinois and fellow Democratic members of Congress. She entered the third quarter with about $2.5 million in the bank.

“She’s calling everyone in [Democratic] politics in Illinois to make sure nothing goes sideways. Call time, Zoom fundraisers, the works,” said one Illinois Democratic strategist. “She’s spooked.”

“Washington Republicans have spent nearly $1.5 million in attack ads against Cheri because she represents a district Donald Trump won and is leading the strategy to keep the Speaker’s gavel out of Kevin McCarthy’s hands,” Denise Mousouris, Bustos’s campaign manager, said in an email. “She always takes her race seriously and we continue to have total confidence in our strategy headed into Election Day. Her focus continues to be on getting real results for the hardworking families she represents, fortifying and growing our Democratic Majority and working to elect Joe Biden and Kamala Harris.”

The National Republican Congressional Committee released a poll earlier this month showing Bustos leading by a slim 49 percent to 44 percent margin. The Congressional Leadership Fund, the top House Republican super PAC, dropped a poll days later that showed Bustos leading King by a 48 percent to 42 percent margin.

The House Majority PAC conducted its own poll earlier this month. That survey showed Bustos leading King too, Democratic sources said, though they declined to discuss details of the survey.

Bustos, running for her fifth term in office, is one of a small handful of Democrats representing districts President Trump won in 2016. Trump carried her district, which runs from the northern border with Wisconsin south along the Mississippi River to Fort Madison, Iowa, and east to Peoria, Ill., by less than a percentage point four years ago.

The Democratic super PAC has spent about twice as much in defense of Bustos as the Republican super PAC has spent against her. The Congressional Leadership Fund has spent about half a million dollars on television and mail pieces against Bustos.

Future45, a super PAC that backs Trump, has spent about a quarter-million dollars on King’s behalf.

The late spending on Bustos’s behalf makes her the 45th candidate for whom the House Majority PAC has spent more than $1 million, along with the most vulnerable Democrats and Republicans in Congress. The Congressional Leadership Fund has spent more on behalf of 52 other Republicans than they have on King, FEC filings show.

A DCCC spokesman declined to comment Saturday morning.

Bustos won the right to chair the DCCC after the 2018 election cycle, when she beat out Reps. Denny Heck (D-Wash.) and Suzan DelBene (D-Wash.) to replace Rep. Ben Ray Luján (D-N.M.), who is now running for a U.S. Senate seat. She is only the second woman to chair the committee.

In a recent interview, Bustos told The Hill that Democrats were positioned to gain seats in November.

“No matter what the national environment looks like, we’ve set ourselves up for wins across the board,” Bustos said. “We keep seeing data and we keep raising the money, and we’ve had a phenomenal, I would say historic, recruitment class.”

Updated: 1:13 p.m.