The commas there are optional, in my opinion.
Student or Learner
From the news:
“I have therefore decided to stand down at the general election.”
Isn’t there too much emphasis put on therefore to omit the commas? On the other hand, it sounds like a strong statement without a pause.
The difference between the almost right word and the right word is really a large matter - 'tis the difference between the lightning-bug and the lightning.
***** NOT A TEACHER *****
I have found some information that may interest you. (It certainly interested me!)
At least two books (that are generally respected) say that the GENERAL RULE is:
If you surround "therefore" with commas, it emphasizes the preceding word. (In the following examples, I have put the preceding words in boldface.)
One expert gives this sentence: "It was John, therefore, who deserved the accolades [expressions of praise]." The author claims that since "John" is emphasized, the sentence implies that the other person who received the accolades did not deserve them.
The author says that this sentence incorrectly puts emphasis on the preceding word: " 'I have heard from residents about firm opposition to the clinic, said [Ms.] Harris, adding that she, therefore, had approached [Mr.] Kearse to reach a resolution [to the problem]."
The author says that "she" is being incorrectly emphasized. Thus, it should be: "...adding that she had therefore approached Kearse to reach a resolution."
Source for all of those quotations: Bryan A. Garner, A Dictionary of Modern American Usage (1998 edition). Besides writing usage guides for ordinary speakers, he also writes book for lawyers, telling them how to write "correct" English for their cases.