The years of GOP leadership have been good ones for energy firms. Oil giant Exxon Mobil posted record annual and quarterly profits last year, while the American Stock Exchange Oil Index has soared over 53 percent since 2000, when Republicans took control of the White House.

Fred Dickson, chief market analyst at D.A. Davidson & Co., says that if Democrats take Congress, expect more regulation and a slowdown in the robust growth the industry has experienced recently.

With Congressional control, says Dickson, Democrats could hamper oil exploration efforts, try to renegotiate leases that are favorable to oil companies, and raise scrutiny about soaring profits at some of the major U.S. oil and gas firms.