Objection 1. It seems that ambition is not opposed to magnanimity by excess. For one mean has only one extreme opposed to it on the one side. Now presumption is opposed to magnanimity by excess as stated above (Question 130, Article 2). Therefore ambition is not opposed to it by excess.
Objection 2. Further, magnanimity is about honors; whereas ambition seems to regard positions of dignity: for it is written (2 Maccabees 4:7) that "Jason ambitiously sought the high priesthood." Therefore ambition is not opposed to magnanimity.
Objection 3. Further, ambition seems to regard outward show: for it is written (Acts 25:27) that "Agrippa and Berenice... with great pomp [ambitione]... had entered into the hall of audience" ['Praetorium.' The Vulgate has 'auditorium,' but the meaning is the same], and (2 Chronicles 16:14) that when Asa died they "burned spices and... ointments over his body" with very great pomp [ambitione]. But magnanimity is not about outward show. Therefore ambition is not opposed to magnanimity.
On the contrary, Tully says (De Offic. i) that "the more a man exceeds in magnanimity, the more he desires himself alone to dominate others." But this pertains to ambition. Therefore ambition denotes an excess of magnanimity.
I answer that, As stated above (Article 1), ambition signifies inordinate love of honor. Now magnanimity is about honors and makes use of them in a becoming manner. Wherefore it is evident that ambition is opposed to magnanimity as the inordinate to that which is well ordered.
Reply to Objection 1. Magnanimity regards two things. It regards one as its end, in so far as it is some great deed that the magnanimous man attempts in proportion to his ability. On this way presumption is opposed to magnanimity by excess: because the presumptuous man attempts great deeds beyond his ability. The other thing that magnanimity regards is its matter, viz. honor, of which it makes right use: and in this way ambition is opposed to magnanimity by excess. Nor is it impossible for one mean to be exceeded in various respects.
Reply to Objection 2. Honor is due to those who are in a position of dignity, on account of a certain excellence of their estate: and accordingly inordinate desire for positions of dignity pertains to ambition. For if a man were to have an inordinate desire for a position of dignity, not for the sake of honor, but for the sake of a right use of a dignity exceeding his ability, he would not be ambitious but presumptuous.
Reply to Objection 3. The very solemnity of outward worship is a kind of honor, wherefore in such cases honor is wont to be shown. This is signified by the words of James 2:2-3: "If there shall come into your assembly a man having a golden ring, in fine apparel... and you... shall say to him: Sit thou here well," etc. Wherefore ambition does not regard outward worship, except in so far as this is a kind of honor.
Source: Summa Theologica (New Advent)