Is it illegal to threaten the life of a candidate for president of the usa?

Yes. However, I'm pretty sure it's against the law to threaten anybody with bodily harm. Actually, TITLE 18 > PART I > CHAPTER 41 > § 871

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(a) Whoever knowingly and willfully deposits for conveyance in the mail or for a delivery from any post office or by any letter carrier any letter, paper, writing, print, missive, or document containing any threat to take the life of, to kidnap, or to inflict bodily harm upon the President of the United States, the President-elect, the Vice President or other officer next in the order of succession to the office of President of the United States, or the Vice President-elect, or knowingly and willfully otherwise makes any such threat against the President, President-elect, Vice President or other officer next in the order of succession to the office of President, or Vice President-elect, shall be fined under this title or imprisoned not more than five years, or both.

(b) The terms "President-elect" and "Vice President-elect" as used in this section shall mean such persons as are the apparent successful candidates for the offices of President and Vice President, respectively, as ascertained from the results of the general elections held to determine the electors of President and Vice President in accordance with title 3, United States Code, sections 1 and 2. The phrase "other officer next in the order of succession to the office of President" as used in this section shall mean the person next in the order of succession to act as President in accordance with title 3, United States Code, sections 19 and 20.