U.S. Senator Lamar Alexander (R-TN) did not seek re-election after eighteen years. He was relatively moderate relative to his caucus (which isn't to say that he hasn't been a fairly reliable vote backing the GOP party line in the Senate) and has been replaced by Former U.S. Ambassador to Japan Bill Hagerty, another Republican who will almost surely be a more reliable conservative than Alexander.
This has provided one more way that it will be harder to craft bipartisan compromises in the U.S. Senate, which he urged his colleagues in the Senate to embrace in his farewell address today. But Majority Leader Senator Mitch McConnell (R-KY) has already repeatedly dismissed. Alexander was typical of the generation of Republican officeholders I grew up with, but unlike some other prominent, but relatively moderate Republicans like Mitt Romney, he did little to oppose Trumpism.
Immediately prior to his retirement, Alexander chaired the Senate Health, Education, Labor and Pensions Committee and was chairman of the Senate Energy and Water Development Appropriations Subcommittee. Both of these committees deal with fewer highly charged partisan social issues than many other committees, were Republicans assigned more solidly conservative leaders. Alexander previously served as governor of Tennessee from 1979-1987. He was also president of the University of Tennessee and U.S. Secretary of Education under George H.W. Bush.