The big money has started to come out in Colorado’s brewing ballot initiative battles.

Two groups on Thursday reported raising more than $3 million apiece last month for their respective campaigns from deep-pockets companies.

The group Coloradans for a Stable Economy, which is fighting a proposed initiative to effectively raise taxes on the oil and gas industry, reported receiving $3.6 million in just five donations. All of the money comes from energy companies, with heavyweights Chevron, EnCana and Williams each shelling out $1 million.

Meanwhile, the group Coloradans for Sensible Solutions, which is backing an initiative to raise the maximum bet at Colorado casinos to $100, reported raising about $3.1 million. Most of that comes from various casinos, with Black Hawk’s Isle of Capri Casino contributing $1.5 million.

To put the monthly hauls for both groups into perspective, Gov. Bill Ritter raised about $4.4 million during the entirety of his 2006 gubernatorial campaign.

Both groups share a campaign strategist, local political consultant Rick Reiter. Dan Hopkins, a spokesman for Coloradans for a Stable Economy, said the overlap is merely a coincidence.

Hopkins said the eye-popping fund-raising numbers for his group show just how concerned the energy industry is about the proposal to take away a tax credit worth about $300 million per year.

That amount, Hopkins said, “is considerably more than what the industry is paying to fight it. It’s a huge tax increase.” Hopkins said consumers would ultimately suffer.

George Merritt, spokesman for the group A Smarter Colorado, which backs the initiative, said his group’s fund-raising report will be ready next week, when most campaigns are expected to file their monthly updates.

“It won’t be three-and-a-half million dollars, I can tell you that,” Merritt said of his group’s fund-raising total. “We said from the beginning this will be a David-and-Goliath struggle.”

Late last month, according to state records, the Sierra Club formed a committee also to push for the proposed initiative. The initiative would put the money it generates into a college scholarship program, as well as several roads, habitat protection and clean energy projects.