The person spoke Tuesday at the baseball winter meetings under condition of anonymity because no deal was announced.
The Nationals had the best record in the majors last season. The NL East champions already have a formidable rotation led by Gio Gonzalez and Stephen Strasburg, and could boost it by adding Haren, an All-Star from 2007-09.
"If we get that finalized, he'll be a great addition," Nationals manager Davey Johnson said. "Haren, he's going to make things better. I think he'll be a leader."
Haren was 12-13 with a 4.33 ERA in 30 starts for the Los Angeles Angels. The Angels nearly traded the 32-year-old righty to the Chicago Cubs for reliever Carlos Marmol after the season, then declined their $15.5 million option on him for next season and paid a $3.5 million buyout.
Haren has made at least 30 starts in eight straight years. Known for his durability, he went on the disabled list for the first time in his career last season because of a back issue.
Johnson said he's familiar with Haren and says he might do even better in the NL.
"The American League is a little tough with the designated hitter," Johnson said.
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