Is writing an article about a document that wasn't leaked yet a crime as severe as actually leaking the document in the United States? Say you know someone from Wikileaks, and you read a classified document that wasn't leaked yet and you make the mistake of posting the news article before the document gets leaked, is this a crime as severe as leaking a classified document?
Under 18 U.S. Code § 798, leaking classified information is a felony, punishable by up to 10 years in prison.
That makes it a more serious offense than publishing legally obtained classified information, which is not a crime at all.
The Supreme Court laid down this rule in Bartnicki v. Vopper, 532 U.S. 514 (2001), where it held that an individual's rights to free speech and free press are generally unaffected by the fact that the information he disseminates was illegally obtained by a third party:
We think it clear that parallel reasoning requires the conclusion that a stranger's illegal conduct does not suffice to remove the First Amendment shield from speech about a matter of public concern.
The answer might be different if you know someone from WikiLeaks because you work there yourself, or if you induced your friend to acquire the information, or if you accessed the WikiLeaks servers without permission. But you haven't indicated any of those are true in the hypothetical, so the answer to your question is no.
Simply "writing an article about a document" isn't any sort of crime in the US, whether the document is or is not leaked, or is or is not classified. However, "disclosing classified information without authorization" can be a crime in some circumstances. If the article discloses classified information from the document, then that might be a crime. For example 18 U.S.C. § 798 provides that:
Whoever knowingly and willfully communicates, furnishes, transmits, or otherwise makes available to an unauthorized person, or publishes, or uses in any manner prejudicial to the safety or interest of the United States or for the benefit of any foreign government to the detriment of the United States any classified information
of various specific types commits a crime. Note that the crime is in making the information available, so discussing information already widely published would not fall under this law.