There's no way this wasn't more true of his presidential run than his senatorial race. He had to raise over a billion dollars to win the presidency and had become very jaded abut politics, from his own words:

And perhaps as the next race approaches, a voice within tells you that you don’t want to have to go through all the misery of raising all that money in small increments all over again. You realize that you no longer have the cachet you did as the upstart, the fresh face; you haven’t changed Washington, and you’ve made a lot of people unhappy with difficult votes. The path of least resistance — of fund-raisers organized by the special interests, the corporate PACs, and the top lobbying shops — starts to look awfully tempting, and if the opinions of these insiders don’t quite jibe with those you once held, you learn to rationalize the changes as a matter of realism, of compromise, of learning the ropes. The problems of ordinary people, the voices of the Rust Belt town or the dwindling heartland, become a distant echo rather than a palpable reality, abstractions to be managed rather than battles to be fought.