Reverend Arthur Dimmesdale has two basic tragic flaws: his feebleness of character and his lack of moral strength.
His feebleness of character is evident, first and foremost, in that he did not resist the temptation of becoming involved with Hester Prynne. This is also indicative of his lack of moral character. If he had both, strength and morality, he would have perhaps even directed Hester in to seeking comfort elsewhere. Similarly, if Dimmesdale had strength of character and a stronger sense of morality he would have forgiven himself for his bad choice and would have accepted the responsibility of what he did, no matter what consequences it would bring.
Instead, Dimmesdale willingly leads a double life where he attempts to moralize his flock while he knows of the sin he has committed. He continues to enjoy his position as an alderman and, although he suffers inside, he does not deviate from fulfilling his role as a celebrated priest. This choice is further evidence of his low moral strength.
So low is Dimmesdale's strength that he pathetically begs Hester for help... to get rid of Chillingworth. The man who once Hester may have seen as strong, intelligent and superior has obviously fallen into decay, all because his strength, intelligence and sense of superiority are all mere appearance.
The judgment of God is on me,” answered the conscience-stricken priest. “It is too mighty for me to struggle with!”
“Heaven would show mercy,” rejoined Hester, “hadst thou but the strength to take advantage of it.”
“Be thou strong for me!” answered he. “Advise me what to do.”
“Is the world, then, so narrow?"
This conversation, which takes place in the forest, shows how the character and strength of Hester towers over Dimmesdale, ultimately rendering him as a weakling in comparison.
Ironically, although it is Hester who has to tolerate the punishment and humiliation of the villagers, she comes out triumphantly as a character while Dimmesdale's choices as a weak man set him aside as what could be deemed a "saint of clay".