Politico:

Sources: officials at NRCC, the House GOP’s campaign arm, had several months’ worth of emails stolen in 2018 by what some think was a nation-state hacker  —  Republican leaders were not informed until POLITICO contacted committee officials about the incident.

Sources: officials at NRCC, the House GOP's campaign arm, had several months' worth of emails stolen in 2018 by what some think was a nation-state hacker (Politico)
Source: Tech Meme