WASHINGTON — Representative Jim Sensenbrenner of Wisconsin, a Republican mainstay who found his way to the center of many of Washington’s most divisive political debates over the past four decades, said on Wednesday that he would not run for re-election next year.

“When I began my public service in 1968, I said I would know when it was time to step back,” Mr. Sensenbrenner, who has represented Milwaukee’s northern and western suburbs in Congress since 1979, said in a statement. “After careful consideration, I have determined at the completion of this term, my 21st term in Congress, it will be that time.”

The announcement adds to what appears to be a growing exodus of House Republicans this summer that promises to change the character of the chamber for years to come, regardless of whether the party wins back control next year. Sixteen lawmakers, including Mr. Sensenbrenner, have said they intend to retire or seek another office in 2020. Just hours earlier, Representative Bill Flores of Texas, said that he, too, would not seek re-election, citing a commitment to term limit himself. And more lawmakers are expected to follow suit.

[Bill Flores became the fifth Republican in Texas to bow out of the next election cycle.]

By contrast, Democrats have seen few retirements so far, though on Wednesday, Representative Susan A. Davis, 75, Democrat of California, said she would not seek re-election in her solidly blue district of San Diego.