The National Republican Congressional Committee discovered this April 2018 that they were hacked. An unauthorized third-party had access to the email accounts of four senior aides. The intrusion was detected in April by an NRCC vendor, who alerted the committee and its cybersecurity contractor. An internal investigation was initiated and the FBI was alerted to the attack. Who these senior aides are has not yet been disclosed. It is not known how the NRCC was hacked, but according to senior party officials, email accounts had been monitored and thousands of emails were exposed.
Source: https://www.bleepingcomputer.com/news/security/national-republican-congressional-committee-hacked-emails-exposed/