Baucus said the formal endorsement of either Sen. Hillary Rodham Clinton (D-N.Y.) or Sen. Barack Obama (D-Ill.) could come as soon as polls close tonight in Montana.
"I expect to endorse tonight," Baucus said."That is, assuming we have a result."
The chairman of the Senate Finance Committee, Baucus is one of the last few undecided Democratic lawmakers.
Other undeclared senators offered few clues which way they were leaning when questioned Tuesday afternoon in the capitol.
"I am going to wait and see what happens in the next 24 hours," said Sen. Jim Webb (D-Va.), who is also uncommitted.
Michigan Democratic Sen. Carl Levin said he was withholding his endorsement with the hopes of seeing his state's delegation fully seated at the upcoming Democratic Convention in Denver.
Delaware Democrat Sen. Tom Carper also demurred when asked if he had made up his mind.