Hitler recognized that in history, the wandering jew had no loyalty to any country and mainly strove to improve himself financially without concern to nationalistic consequences. This factor, among others, interfered with his hope to unify Germany into a great power.
So, again addressing your second question, his power and assumed authority to persecute the Jews grew as his charismatic personality enhanced his ability to command a following among others who joined him in viewing Jews as the source of all the economic and social difficulties of that time. He, and they, did not see what was being planned and carried out as an "overgrowth" of power but as the process needed to protect and purify the German homeland and the mother race.
Jews were an easy and very traditional target for most Europeans, as anti-Semitism had been common across most of Europe since the diaspora centuries earlier. By inflaming hatred against this minority, it became easiuer for Hitler to attain power and to hold it. For Hitler, it was not just political opportunity, but genuine hatred on his part for the Jewish people. Hitler had been raised and formed his opinions in the same anti-Semitic environment as other Europeans, so he was a product of that hatred.
Hitler feared the Jews' financial power, and he believed that genetically mixing with the Jews made Germans impure. He saw them as both a threat and as inferior humans. As Hitler's power and military superiority grew, he stepped up his extermination plans for the Jews since there was no one to stop him.
The other answers are very detailed, but just to reiterate, yes Hitler used the Jews as a scapegoat. He personally was a bit unstable, and in his paranoid mind they were a threat. The main reason he targeted them was that there was already so much prejudice against them that it was easy to turn people on them. By targeting the Jews he united people in an us against them crusade, and that definitely cemented his power. People feared him and the party, and were generally happy it wasn't them on the chopping block.
It is probably an overstatement to state that Hitler considered the Jews a threat; otherwise he would have dealt with them more aggressively in the beginning. He rather considered them a useless nuisance, akin to vermin that the country would do well to be rid of. In Mein Kampf he compares them to "maggots."
Was there any excrement, any shamelessness in any form, above all in cultural life, in which at least one Jew would not have been involved? As soon as one even carefully cut into such an abscess, one found, like maggots in a decaying body, often blinded by the sudden light, a kike
His initial policy as Chancellor was to isolate them, encouraging the German people to not do business with them, and later depriving them of practicing certain professions. In terms of ridding the country of them, he first considered exporting all German Jews to Israel, but was thwarted by the British who were afraid of an Arab rebellion if the Jewish population increased. He next considered shipping them to Madagascar, at that time a French colony; but the French resisted also. Finally, he considered exporting them to the far reaches of the Soviet Union. Only when all of these "remedies" proved impractical did the "Final Solution" rear its ugly head.
Although Hitler used anti-Semitism in his campaign to secure power for the National Socialist Party, his power did not grow as a result of his actions toward the Jews after his election as Chancellor. Under the Constitution of the Weimar Republic, he had already been granted dictatorial powers under the Emergency Powers provision following the burning of the Reichstag. At that point, his power was complete. His policies toward the Jews were an exercise of that power, nothing more.
It should also be noted that anti-semitism was not limited to Germany. When German troops "liberated" Poland, many Polish people attacked Jews in the streets, killing many of them by hitting them with broomsticks which had nails embedded in them. Hitler's actions were largely a reaction to a pre-existing attitude toward European Jews.
Hitler also felt that Jews were simply inferior. It was not just that they were different from "real" Germans. His ideology held that Jews were an inferior race that could not create/improve a civilization but could only destroy it. He called them a "culture-destroying" race. Because of this, he claimed that Jews were a threat to Germany and needed to pushed out of (eventually in the most horrible way imaginable) German society.
Allow me to address the second question.
Many people who start "hate groups" begin with a very limited power over others. As people who hold similar beliefs join together, the power of the group grows substantially. This is not to say that Hitler did not have power, but it seems that the "movement" had a greater power. Would Hitler admit that he had become too powerful in regards to the oppression and murder of the Jewish race? I would have to say no.
Instead, i would tend to believe that any person whose desire lies in control and power over all would never admit to being too powerful.
Well, let me start the ball rolling by responding to your first question. Hitler saw the Jews as a threat because he felt that they represented, amongst other things, a threat to the powerful nationalistic spirit he was trying to create. Jews, Hitler felt, because they have a deeper identity that is racial, cannot be linked to any one nation, as their blood gives them a more powerful unity with others of their kind than they could ever feel towards a country. Because of this, they were a threat against the Reich and the glories of the Fatherland.