Rep. Fred Upton speaks as challenger Mike O'Brien listens during a debate on Oct. 8. (Matt Gade/AP)

In an election dominated by negative advertising, one group of big spenders is determined to stay positive. Industry trade associations, from dentists to credit unions, are pumping money into races and spending it overwhelmingly on feel-good messages.

When Rep. Fred Upton (R-Mich.) faced a primary challenger this spring, he had the American Chemistry Council, the Realtors, radiologists and optometrists behind him. They sponsored television ads, direct mail and Web sites telling Upton’s constituents what a great job he’s done.

Officials at trade associations, which tend to be more bipartisan than other interest groups, say they’re glad to be doing things differently.

“We feel that there’s a need for some positive discussion in Washington about some of the leadership that’s been shown there,” said Anne Kolton, a spokeswoman for the chemistry council. “There’s value in highlighting good work on important issues.”

In Upton’s case, supplying the margin of victory may not have been the objective as much as winning favor with the 12-term incumbent. Upton is chairman of the powerful House Energy and Commerce Committee, which has oversight over vast swaths of the economy.

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In fact, he didn’t need much support: His campaign outspent his challenger’s by 20 to 1 and he received twice as many votes in the primary. The roughly $200,000 spent by the trade groups was less than one-tenth of Upton’s campaign spending.

Political spending by industry associations has shot up — $8 million was spent through September, with all of it going for positive ads, according to documents filed with the Federal Election Commission. That is double the figure in each of the previous two campaigns and 10 times the 2006 spending.

The trade associations are spending more following a landmark 2010 Supreme Court decision and other rulings easing restrictions on corporate political spending.

The behemoth U.S. Chamber of Commerce, which serves as an umbrella group for business interests, also has spent heavily on the election, but mainly on attacks against Democratic lawmakers. And most of the spending by interest groups and super PACS that have formed to support candidates from both parties has been on negative ads.

There was not a hint of negativity when the American Chemistry Council, which represents large chemical manufacturers, ran an ad supporting Upton in the primary.

“From the shores of Lake Michigan to our factories and family farms, hard work is a way of life,” a narrator says. “And in Washington, Congressman Fred Upton is working hard for us to cut red tape and government regulation.”

The group, which has pledged to spend several million dollars on ads for its favored lawmakers, praised in a news release Upton’s support of “a comprehensive energy policy, pro-growth tax policies and rational science-based regulation.”

Jamie Gregory, chief deputy lobbyist for the National Association of Realtors, said his group doesn’t spend on a race if the help is not needed. “We get involved in races to make a difference, not to just wave a flag and make somebody feel good,” Gregory said.

The Realtors have spent the most in two California races, devoting $1 million to help Rep. Brad Sherman (D) and $1.6 million for Rep. Gary G. Miller (R). Both have supported the Realtors’ legislative priorities, Gregory said.

The group is focused on preserving preserving the mortgage interest deduction and the pending reform of mortgage giants Fannie Mae and Freddie Mac.

But more of the Realtors’ money goes to state and local races, where decisions on zoning and environmental concerns affect development.

Federal law bars corporations, unions and other interest groups from donating directly to candidates, but they have traditionally sponsored political action committees to funnel employee donations in amounts limited to $5,000. That still accounts for most of the corporate-linked money flowing to federal races.

In 2010, the Supreme Court’s ruling in Citizens United v. Federal Election Commission struck down limits on corporate or interest group spending to support candidates, as long as the groups don’t coordinate with the candidates.

“We couldn’t do this without Citizens United,” said Peter Kezirian, an executive with the Cooperative of American Physicians, which provides insurance for California doctors and advocates for tort reform.

The cooperative, which functions in some ways like a professional association, has been involved in state and local races for years under California law. This election cycle it is including federal races through a super PAC with $2.6 million from the cooperative’s general treasury.

The group runs positive ads because the doctors who run it are “men and women of science,” Kezirian said, adding that the spots help lawmakers by boosting their favorability with voters. “It allows them to withstand the negative hits that are going to come,” Kezirian said.

Professional groups often reserve the biggest sums to benefit members of the trade. Ophthalmologists have spent $153,000 — two thirds of their ad spending — to benefit Rep. Nan A. S. Hayworth (R-N.Y.). Dentists are strongly pushing Reps. Mike Simpson (R-Idaho) and Paul A. Gosar (R-Ariz.), both fellow dentists.

“They can be champions not just of our bills, but champions of dentistry within Congress,” said William Calnon, president of the American Dental Association and a practicing dentist in Rochester, N.Y.