In the Democratic contest, John Edwards and Mrs. Clinton were unwilling to confront him [Obama] directly or in a manner that hurt him. Mr. McCain was rightly preoccupied by his own primary. On Tuesday night, things changed.

Perhaps in response to criticisms that have been building in recent days, Mr. Obama pivoted Tuesday from his usual incantations. He dropped the pretense of being a candidate of inspiring but undescribed “post-partisan” change.

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.