WASHINGTON — President Obama plans to nominate Senator Max Baucus, the Montana Democrat and chairman of the Senate Finance Committee, as his next ambassador to China, according to three officials familiar with the pending announcement.

Mr. Baucus, the chief Capitol Hill architect of Mr. Obama’s health care overhaul, had already decided not to seek re-election next year. He has served six terms and is Montana’s longest-serving senator. He said in a recent interview that he was not certain of his plans, but was looking forward to “a whole new adventure, a whole new chapter.”

Known as an independent thinker, Mr. Baucus has long made Democrats uncomfortable. He broke with the party as one of the writers of President Bush’s tax cuts in 2001, and again to support a Medicare prescription drug benefit in 2003. He aroused the ire of Democrats with his protracted — and unsuccessful — effort to get Republican support for the health care bill, which he regards as one of his major accomplishments.

Mr. Baucus waved off reporters in the Capitol on Wednesday, declining to comment on the offer. “It is not for me to discuss,” he said.