In a sense he was what is called a gentleman - Trollope's long concern with con men

Trollope had been working on con artist-like characters at least since Framley Parsonage (1861) – maybe since Barchester Towers (1857) if Slope counts – but the explicitly public face of the Palliser novels concentrated his attention.  The con man in Can You Forgive Her? (1865) briefly becomes a Member of Parliament.  The most prominent one in The Way You Live Now (1875) – not a Palliser novel but written amidst them – creates a financial bubble.  Lizzie Eustace, the greatest of them all (The Eustace Diamonds, 1873), is merely a celebrity, I guess, but one reason she is the best is because she knows how to ride over difficulties to get to the next con.  Also, she’s enormous fun.

Ferdinand Lopez is, in this company, incompetent.  He peaks early in The Prime Minister, creating enough glamour and smoke to cause a lot of damage, especially to Emily Wharton, who marries him, and, by a chance shot, to the Prime Minister of England.  Lopez tries to get into Parliament, but fails.  He tries to corner a market, but fails.  He marries wealth, but fails to get his hands on much of it.

He believed in the guano and he believed in Mr. Wharton, but it is a terrible thing to have one's whole position in the world hanging upon either an unwilling father-in-law or a probable rise in the value of manure! (Ch. 25)

The bit I bolded was surprisingly earthy, and direct.  Lopez’s value is directly tied to the value of manure.  And at least manure is good for something – it is another speculation, in kauri gum, a “substitute” for amber, that really does him in.  The other nice point in this quotation is the word “believed,” which is what makes him a real, if mediocre, confidence artist.  He believes in his own con, at least at any given moment, as necessary.

“To give him his due, he did not know that he was a villain” (Ch. 54).  That line comes early in a great chapter where Lopez tries to work his magic on Lizzie Eustace, not knowing, apparently, that you can’t con a con.  At least a worse con won’t beat a better.  The chapter is humorously ironic for readers of The Eustace Diamonds and likely a little baffling for others, but who is reading The Prime Minister who has not read The Eustace Diamonds?

The con man character, in its male form, is a distillation of Trollope’s long-running critique of and unease with the idea of the “gentleman.”  Trollope cleverly misdirected me early in the novel by emphasizing the prejudices of Lopez’s enemies – that Lopez is foreign (a Portuguese father), or that he is Jewish (without evidence) – prejudices that are all too gentlemanly.  Only later does he reveal that Lopez is a sham.

In a sense he [Lopez] was what is called a gentleman. He knew how to speak, and how to look, how to use a knife and fork, how to dress himself, and how to walk.  But he had not the faintest notion of the feelings of a gentleman. (Ch. 58)

The Duke, the Prime Minister, the other male protagonist, is the epitome of the English gentleman.  In his story, it is the strength of his gentlemanly feelings that cause him to suffer, that in fact provide his side of the novel’s plot.  Trollope finally, in The Prime Minister, draws a line.  The gentleman should be like this to prevent that – the rise of the Lopezes.  We need the standards.  We are too easily fooled.