Baucus is a Democrat and chairman of the Senate Finance Committee, which oversees trade issues. The 72-year-old was first elected to the Senate in 1978.
Baucus has said he wants to help build a better economic relationship with China. The US trade deficit with China is far bigger than it is with any other country, and China is the largest foreign holder of US debt.
China also has a growing military and a government that US officials say has a poor human rights record.