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Mr. Smith’s campaign took a similar tack.

“What’s shocking isn’t that Tom Smith is for common-sense, pro-American energy policies, rather that [Sen.] Casey has supported the Obama administration’s war on American-made energy,” spokeswoman Megan Piwowar said.

In North Dakota, Heidi Heitkamp, a Democratic candidate for Senate, has at least $100,000 in Vanguard Energy, a mutual fund invested in energy producers that netted her at least $5,000 in dividends last year, records show.

She also received nearly $20,000 in compensation last year as a board member of the Dakota Gasification Co.

Tens of thousands of jobs have been created in North Dakota, much like Pennsylvania, in recent years because of an explosion in shale drilling, placing it on the forefront of the nation’s energy boom.

Ms. Heitkamp’s opponent, Republican Rick Berg, also holds at least $36,000 in energy investments, including stock with Hess Corp. and Occidental Petroleum Corp.

Many others — such as Senate hopeful Bob Kerrey of Nebraska and Republican Ted Cruz, who will face Mr. Dewhurst in a July 31 runoff election in Texas — also have thousands of dollars in the fossil-fuels sector.

In West Virginia, the U.S. Senate race is between two men who have made fortunes largely through the energy industry. Incumbent Joe Manchin, a Democrat, has a stake of between $1 million and $5 million in Enersystems Inc., a coal company he used to run. His son now controls the company, but it still paid Mr. Manchin at least $800,000 last year, records show.

His Republican opponent, John R. Raese, made more than $1.5 million last year at Greer Industries, a company with interests in coal, lime and a variety of other ventures. He also holds tens of thousands of dollars in investments related to gold and other mined minerals.

There are, however, a few notable exceptions to the trend. Republican Deb Fischer, who will face off against Mr. Kerrey in Nebraska, holds no energy investments. Neither does Mr. Brown of Ohio.

While it’s difficult to know whether their personal finances would directly influence decision-making, Mrs. Sloan said, the cross-section of bank accounts and policymaking justifiably erodes the public’s trust in their elected officials.

“The appearance [of impropriety] is bad enough,” she said.