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 NelsonBill NelsonSenate advances Trump's Commerce pick CMS nominee breezes through confirmation hearing Net neutrality fix faces hard sell MORE (Fla.), Jon TesterJon TesterGOP loses top Senate contenders Senate advances Trump's Commerce pick Schumer tries to keep the peace as Sanders speaks out MORE (Mont.), and Sherrod BrownSherrod BrownHouse bill would prevent Trump from lifting Russian sanctions Dem senators call for independent Flynn probe Overnight Regulation: Trump signs repeal of oil industry transparency rule MORE (Ohio) as well as Rep. Tammy BaldwinTammy BaldwinGOP loses top Senate contenders Duffy not running for Senate in 2018 Got soy milk? Don't let Congress, dairy industry bogart 'milk' label MORE (D-Wis.) for backing the Democratic health insurance reform law, and slam Rep. Martin HeinrichMartin HeinrichDem senator calls out Trump for leaving key to apparent classified info exposed Trump’s pick for CIA No. 2 prompts Dem fears Intel Committee Dems to Trump: Read torture report 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: