WASHINGTON  When Cooper Industries, a century-old manufacturing company based in Texas, moved its headquarters to Bermuda to slash its American income tax bill, it had to turn to a Washington insider with extraordinary contacts to soothe a seething Congress.

Dan Coats, then a former senator and ambassador to Germany, served as co-chairman of a team of lobbyists in 2007 who worked behind the scenes to successfully block Senate legislation that would have terminated a tax loophole worth hundreds of millions of dollars in additional cash flow to Cooper Industries.

Now Mr. Coats, a Republican from Indiana, is about to make a striking transition. He is spinning the revolving door backward.

As part of the Republican wave in this year’s midterm elections, Mr. Coats will join the Senate again and is seeking a coveted spot on the Finance Committee, the same panel that tried to shut the tax loophole and that the Obama administration has pushed to again consider such a move.