He adopted that "obligation" because of political and economic interests in the union's continuity itself, prominent among those interests being personal political power and economic protectionism. But rather than concede other interests, he conducted his campaign against secession around what was, as Tocqueville put it 30 years earlier, the "borrowed name" of the maintaining the Union per se. That is where the deception lies.
I don't believe he had any intentions to do so, with the purpose of becoming a dictator and overthrowing the Constitution.
Intentions and their actualization are two entirely different things.