WASHINGTON — Senator Bob Corker, Republican of Tennessee and chairman of the Senate Foreign Relations Committee, announced on Tuesday that he would not seek re-election next year, a move that is likely to set off a scramble pitting mainstream Republicans against President Trump’s populist wing.

Mr. Corker, 65, an establishment Republican who joined the Senate in 2007, would probably have drawn a primary challenge from the right — and hinted as much in his statement announcing his retirement, which said he had always been drawn to the idea of a “citizen legislator.”

“I also believe the most important public service I have to offer our country could well occur over the next 15 months,” Mr. Corker said, “and I want to be able to do that as thoughtfully and independently as I did the first 10 years and nine months of my Senate career.”

Mr. Corker’s decision not to run again creates the first open Senate seat of the 2018 election cycle. Jennifer Duffy, an analyst with the nonpartisan Cook Political Report, predicted Republicans would retain the seat. But, she said, they “will have a free-for-all in terms of the nomination.”