Until now, Mr. Obama has been making appeals to the center, saying, for example, that we are not red or blue states, but the United States. But in his Houston speech, he used the opportunity of 45 (long) minutes on national TV to advocate a distinctly non-centrist, even proudly left-wing, agenda.

By doing so, he opened himself to new and damaging contrasts and lines of criticism.

Mr. McCain can now question Mr. Obama’s promise to change Washington by working across party lines. Mr. Obama hasn’t worked across party lines since coming to town.

Was he a member of the “Gang of 14” that tried to find common ground between the parties on judicial nominations?

Was Mr. Obama part of the bipartisan leadership that tackled other thorny issues like energy, immigration or terrorist surveillance legislation? No. Mr. Obama has been one of the most dependably partisan votes in the Senate.

Mrs. Clinton can do much more to draw attention to Mr. Obama’s lack of achievements. She can agree with Mr. Obama’s statement Tuesday night that change is difficult to achieve on health care, energy, poverty, schools and immigration—and then question his failure to provide any leadership on these or other major issues since his arrival in the Senate. His failure to act, advocate or lead on what he now claims are his priorities may be her last chance to make a winning argument.

Mr. McCain gets a chance to question Mr. Obama’s declaration he won’t be beholden to lobbyists and special interests. After Mr. Obama’s laundry list of agenda items on Tuesday night, Mr. McCain can ask why, if Mr. Obama rejects the influence of lobbyists, has he not broken with any lobbyists from the left fringe of the Democratic Party? Why is he doing their bidding on a range of issues? Perhaps because he occupies the same liberal territory as they do.

The truth is that Mr. Obama is unwilling to challenge special interests if they represent the financial and political muscle of the Democratic left. He says yes to the lobbyists of the AFL-CIO when they demand card-check legislation to take away the right of workers to have a secret ballot in unionization efforts, or when they oppose trade deals. He won’t break with trial lawyers, even when they demand the ability to sue telecom companies that make it possible for intelligence agencies to intercept communications between terrorists abroad. And he is now going out of his way to proclaim fidelity to the educational unions. This is a disappointment since he’d earlier indicated an openness to education reform. Mr. Obama backs their agenda down the line, even calling for an end to testing, which is the only way parents can know with confidence whether their children are learning and their schools working.

These stands represent not just policy vulnerabilities, but also a real danger to Mr. Obama’s credibility and authenticity.

He cannot proclaim his goal is the end of influence for lobbies if the only influences he seeks to end are lobbies of the center and the right.