Complex sentence: The optimates were also weary of the personal political clout of Pompey and saw him as a potential challenge to the supremacy of the senate, which they largely controlled and which had been put into question with the criticism to the summary executions during the Catilinarian conspiracy.
Split into simpler sentences: The optimates were also weary of the personal political clout of Pompey. They saw him as a potential challenge to the supremacy of the senate, which they largely controlled and which had been questioned with criticism of summary executions during the Catilinarian conspiracy.