Sens. Claire McCaskillClaire Conner McCaskillManchin used Heimlich manuever on McCaskill during caucus luncheon Nail manufacturing exec who voted for Trump blames him for layoffs, asks Democrat for help The American economy is stronger than ever six months after tax cuts MORE (D-Mo.), Charles SchumerCharles (Chuck) Ellis SchumerSchumer calls for Trump administration to appoint 'czar' to oversee family reunification Donald Trump Jr. headlines Montana Republican convention Montana's environmental lobby teams with governor to kill 600 jobs MORE (D-N.Y.), Dick DurbinRichard (Dick) Joseph DurbinSenate left in limbo by Trump tweets, House delays The Hill's Morning Report — Sponsored by PhRMA — Immigration drama grips Washington Senate Gang of Four to meet next week on immigration MORE (D-Ill.) and Patty MurrayPatricia (Patty) Lynn MurrayOvernight Health Care — Presented by the Association of American Medical Colleges —Dems, health groups demand immigrant children be quickly reunited with families White House releases sweeping proposal to reorganize government Democrats protest Trump's immigration policy from Senate floor MORE (D-Wash.) cited press reports that this may be happening and told FTC Chairman Jon Leibowitz that this would be a "direct affront" to American consumers.

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"At a time when major refiners and oil companies are making record profits and American families continue to struggle with gasoline at record prices, the idea that refiners may be manipulating the market to keep prices artificially high is offensive," they wrote.

"It is incumbent upon the Commission to ensure that the American people are protected from this type of manipulation," the letter continued. "Accordingly, we request that the Commission open a full investigation into these allegations of wrongdoing and to determine the impact this behavior, if confirmed, has on regional and national gasoline prices."

The letter acknowledged that rising oil prices are "certainly a driving factor" behind gas price increases. However, it added that U.S. gasoline use is down, but gasoline inventories are also low, and refineries are using only 81.7 percent of their capacity, down 7 percent from last year.

They also said refiners have seen a 90 percent increase in their margins. "While some have argued that this increase is due to potential impacts from recent flooding along the Mississippi River, this cannot justify the steady increases in their margins since January of this year," they wrote.

McCaskill noted the letter on the Senate floor Tuesday, just before a planned evening vote to end debate on a bill that would raise taxes on oil companies.