1) He was intentionally disingenuous: He voted for the amendment in order to cast himself as a defender of the faiths, especially Catholicism, and denied facts about the amendment's scope in order to preserve his image as a pro-choice, Massachusetts moderate. (As a state senator, Brown voted for a contraceptive coverage mandate that he now opposes.)
2) He didn't read the amendment before voting for it and was unfamiliar with its exact language; or 3) He didn't understand the amendment and thought the language referencing "moral convictions" didn't matter. In other words, Brown is either a dishonest politician of questionable character or a negligent or dim-witted one of questionable competence. Personally, I don't imagine that he's stupid.
Should voters who support Brown's policy agenda, assuming they can accurately discern it, care if he's a flawed character? (Isn't stonewalling all in the game?) Should Warren's political allies care if she too has less integrity than they imagined? For the sake of argument, assume the worst about both candidates. Assume that she claimed her fabled Native American heritage to gain an advantage in the Ivy League hiring process. Assume that he misrepresented his vote on the Blunt Amendment for show, in order to cast himself as a moderate, pro-choice defender of religious liberty and that he poses as a champion of working and middle class consumers while quietly advancing the interests of Wall Street and too big to fail banks.
But if Brown's posturing on religious liberty, choice, and financial reform is fair game in a race focused on character, it seems unlikely to affect his high favorability ratings or assessments of his integrity among target voters. His fumbles on the Blunt Amendment provided fodder for liberal blogs, but undecided and swing voters may have found his rhetoric about religious liberty persuasive. Even relatively simple legislation like this is relatively complicated in the context of a campaign, and readily spun.
Financial reform is particularly complex. Facts about Brown's public role in passing Dodd Frank and his private role in helping to gut it can be easily obscured in the fog of political war. He can successfully dispute the Globe story with a general denial of its accuracy and a claim that he's focused on job creation. Voters who like Brown and have already credited him with good character are likely believe him.
How do less partisan voters evaluate character? I suspect they rely on personal stories, not policy debates, regardless of what the framing of those debates -- and a candidates' forthrightness in explaining his positions -- reveal about character. In Massachusetts, the appeal of the personal over the political remains a formidable challenge for Elizabeth Warren. Facts about financial reform are a lot less engaging, a lot harder to dramatize, personalize, and mock than Warren's claim of Native American ancestry.
The morning after the front page Boston Globe story on Brown's "loosening" of banking regulations, a local NPR affiliate (WBUR) ran yet another review of the fracas over Warren's heritage and its effect on the Senate race. The report of Brown's record on financial reform wasn't even mentioned.