WASHINGTON—Members of the 2016 Trump campaign represented a major counterintelligence risk to the U.S. due to their frequent contacts with individuals with close ties to the Russian government, a bipartisan Senate investigation has concluded.

The Senate Intelligence Committee released the fifth and final volume of its Russia investigation report Tuesday. The partially redacted document is nearly 1,000 pages and largely supports the key findings on Russian election interference made by former special counsel Robert Mueller,…