This is the latest volley from the group. It spent a combined $1.4 million on Senate races in Hawaii, Nevada, New Mexico and North Dakota earlier this month.

The ads hit Democratic Sens. Bill NelsonClarence (Bill) William NelsonSenate campaign fundraising reports roll in Week ahead: Tech giants to testify on extremist content Puerto Rico's children need recovery funds MORE (Fla.), Jon TesterJonathan (Jon) TesterEMILY’s List president: Franken did 'right thing for Minnesota' Reforming veterans health care for all generations of veterans Trump and Republicans deliver gift that keeps on giving for Americans MORE (Mont.), and Sherrod BrownSherrod Campbell BrownSenate campaign fundraising reports roll in Commerce sends Trump long-awaited steel report GOP Rep. Jim Renacci announces Ohio Senate bid MORE (Ohio) as well as Rep. Tammy BaldwinTammy Suzanne BaldwinSenate campaign fundraising reports roll in Listen: EMILY’s List upbeat about Dem House in '19 Bolton to spend M boosting Wisconsin Senate candidate MORE (D-Wis.) for backing the Democratic health insurance reform law, and slam Rep. Martin HeinrichMartin Trevor HeinrichTrump, GOP fire back over Fusion GPS testimony Overnight Cybersecurity: Computer chip flaws present new security challenge | DOJ to offer House key documents in Russia probe | Vulnerability found in Google Apps Script Overnight Cybersecurity: Senators unveil election security bills | North Korea denies WannaCry role MORE (D-N.M.) for opposing expanded drilling for oil and immediately building the Keystone XL pipeline.

The group does not disclose how much it spends on ads and did not respond to a request for comment on this ad buy.

The Democratic Senatorial Campaign Committee fired back, accusing the Chamber of spending "secret money" to prop up "weal candidates."

"These Republican candidates would be nothing more than a rubber stamp for the Chamber’s special interest agenda that puts insurance companies over seniors, oil companies over small businesses, and billionaires over the middle class," said DSCC spokesman Matt Caner. "As voters learn more about the Republicans' special interest agenda, they'll quickly realize how wrong these priorities are for their families and our country."

Watch the ad against Heinrich: