[House Atreides]

I think the position will be vacant until filled in November’s general election. So this vile creature (tool of the GOP establishment) does damage to Trump’s agenda on his way out. He could have retired effective in calendar year 2017 and had the vacancy filled by a special election,

[theoilpainter]

This a real blow to Trump. It will make getting his judges confirmed one vote harder.

[SMGFan]

Possibly
“Well, it took a little longer than we expected, but Sen. Thad Cochran (R) is finally calling it quits. The 80-year old has been in poor health for the last six months and it was considered unlikely that he would stick things out until 2020. By resigning now, the seat will be filled in the November general election, though I believe it will still be decided in a non-partisan Louisiana Rules Top Two Format”

https://rrhelections.com/index.php/2018/03/05/ms-sen-sen-thad-cochran-r-resigning-april-1/