WASHINGTON—The Senate Intelligence Committee is drafting a report on vulnerabilities in the U.S. election system that is expected to be released in the coming weeks, according to lawmakers and aides, in what will be the first product of a yearlong investigation into Russian interference in the 2016 election.

The leaders of the Senate Intelligence Committee said they hope to have it completed by March. The report still needs to be vetted and approved by multiple federal agencies to ensure the protection of classified information,...