Machiavelli says that when Roman senators (almost exclusively patricians) saw that only plebeians were in the running for consular tribune, they maintained patrician monopoly on power in one of two ways:

1. They convinced a distinguished and well-regarded patrician to run. Citizens would be reluctant to withhold consular powers from such a noble personality: they would be "ashamed to refuse".
2. They bribed a low-minded plebeian to join those plebeians who were already seeking the office. Citizens would recoil from giving such a person consular powers, and through guilt by association, they would be reluctant to extend consular power to any plebeian: they would be "ashamed to give".

A different translation puts matters somewhat more clearly:

Chapter XLVIII

Whoever wants a magistracy not to be given to a vile or wicked one, will have it asked by a man more wile and more wicked, or by one more noble and more good.

When the (Roman) Senate became apprehensive that the Tribunes with Consular power should be created from plebeian men, they took one of two courses: either they caused the more reputable men of Rome to be designated, or by suitable means they (surely) corrupted some sordid and most ignoble Plebeians, who mixed with the plebeians of better quality who usually asked for these offices, so that even they should ask for them. This latter course caused the Plebs to be ashamed of themselves to give it to the latter, and the first (course) made them ashamed to take it away from the former. All of which confirms the proposition of the preceding discussion, where it is shown that the people deceive themselves in general matters, but they do not deceive themselves in particular matters.

_{Machiavelli, Niccolò. Discourses on Livy. c. 1517. Modernized version of 1695 translation by Henry Neville. Open source. Accessed at archive.org 27 December 2023. p. 110.}

Machiavelli's original Italian for the word translated as wicked is tristo:

Cap. XLVIII

Chi vuole che uno magistrato non sia dato ad un vile o ad un tristo, lo facci domandare 0 ad un troppo vile e troppo tristo, o ad uno troppo nobile e troppo buono.

Quando il Senato dubitava che i Tribuni con potestà consolare non fussino falli d'uomini plebei, teneva uno de'duoi modi: o egli faceva domandare ai più riputati uomini di Roma; o veramente, per i debiti mezzi, corrompeva qualche plebeio sordido ed ignobilissimo, che mescolati con i plebei che, di miglior qualità, per l'ordinario lo domandavano, anche loro io domandassino. Questo ultimo modo faceva che la Plebe si vergognava a darlo; quel primo faceva che la si vergognava a tòrlo. Il che tutto torna a proposito del precedente discorso, dove si mostra che il popolo, se s'inganna de'generali, de' particolari non s'inganna.

_{Machiavelli. Niccolò. Discorsi sulla prima deca di T. Livio. Scelti e commentati da Giuseppe Piergili. Firenze: Successori Le Monnier, 1892. pp. 183–184}

From the Collins Italian-English Dictionary it can be seen that while wicked is indeed an accurate translation, so is poor, mean, or sorry. The ambiguity allows Machiavelli to use the word in both senses. In the first instance, tristo refers primarily to someone who is mean, in the sense of ordinary or not distinguished: a plebeian, say. In the second, tristo means evil. In order to prevent someone who was mean in the sense of ordinary (i.e., a plebeian) from being elected, the patricians would associate that person with another plebeian, someone mean in the sense of wicked. Livy's History of Rome Book IV, Chapter 56 does specify precisely this stratagem:

At Rome, as the commons gained the victory, so far as to procure the kind of election which they preferred, so in the issue of it the patricians were victorious: for, contrary to the expectations of all, three patricians were chosen military tribunes with consular power: Caius Julius Iulus, Publius Cornelius Cossus, and Caius Servilius Ahala. It is said that an artifice was practiced by the patricians on the occasion, and the Icilii charged them with it at the time: that by intermixing a number of unworthy candidates with the deserving, they turned away the people's thoughts from the plebian candidates. The disgust was excited by the remarkable meanness of some of the number.

_{Livy (Titus Livius). Livy in Five Volumes. Trans. by George Baker of Ab Urbe Condita, c. 27 BCE – c. 9 BCE. Vol. 1. New York: Harper, 1844. pp. 302–303.}

So the goal is not so much to prevent a wicked man from being elected; it is to maintain the monopoly of the established ruling classes. The semantic association of tristo with both evil and a lack of distinction leads to a similar muddying of the waters when a genuinely evil person of the plebeian class is encouraged to associate himself with other plebeians whose only fault is that they do not belong to the distinguished class of patricians. The result is that the patricians maintain their lock on political power.

_{Note. With the exception of the dictionary link, all links are to texts on archive.org. All links, including the dictionary, are live as of 28 December 2023.}