Baucus is the 72-year-old the Senate Finance Committee chairman, now in his 36th year in the Senate.
The Montana Democrat has played a role in China trade issues, but he has no real expertise in military, human rights and other issues that have caused tensions in recent years between the US and Beijing.
Baucus had planned on retiring from the Senate after his term expires in January. His early departure could give Democrats some help in retaining his seat.
That's because Montana's Democratic governor is expected to appoint a Democrat to replace Baucus, letting that person establish a record and enjoy visibility if they run for the seat in November.