Booker's major substantive difference with many progressives is on education policy. He is -- like President Obama -- an advocate of the "education reform" movement; he has backed New Jersey Governor Chris Christie's expansion of charter schools and merit pay for teachers, as well as a form of vouchers for some impoverished areas. He sits on the board of Democrats for Education Reform. During last summer's Democratic convention, Booker spoke at an event hosted by lightning-rod former D.C. schools chancellor Michelle Rhee, who teachers unions see as working to privatize public education and undermine collective bargaining. The school-reform issue is the subject of a major schism in today's Democratic Party; Obama's "Race to the Top" education initiative, which has encouraged state-level reforms, has infuriated traditional Democratic allies but also drawn support from many party officeholders.

Booker has also been deemed suspicious when it comes to entitlement reform. The sole concrete criticism in The New Republic's recent takedown was an allusion to Booker "hinting that he'd be open to raising the Social Security retirement age for young people -- before backtracking furiously when progressives called him on it." Booker had been paraphrased in the Bergen Record as saying that he "opposes raising the retirement age for most people in the country -- except, perhaps, for people in their 20s or younger." When the vagueness of that position prompted furious criticism, Booker tweeted that he opposes all cuts to Social Security and Medicare; would, if anything, expand the programs; and also opposes raising the retirement age and curbing benefits through the "chained CPI" inflation index.

But the case against Booker seems to rest chiefly on tone and approach. Like Obama, he has positioned himself as a conciliator willing to work across the aisle; like Obama, he is a black politician who has attempted to transcend racial divisions and has challenged inner-city black machine politics. Progressives disdain the spectacle of Booker's friendship with Christie -- another pol who, despite mostly doctrinaire policy stances, has found himself at odds with his party's base for palling around with the other side (in Christie's case, with President Obama). Obama, for his part, is said to see his younger self in Booker. Endorsing his Senate bid in a statement this week, the president said, "Cory Booker has dedicated his life to the work of building hope and opportunity in communities where too little of either existed."

Booker's allies find it perplexing that normal politician activities, like raising money and being ambitious, are seen as uniquely damning in his case. They insist that he is sincerely motivated by issues of inequality and social justice; the first policy paper he released during the current campaign was about child poverty, hardly a political winner. The hobnobbing with billionaires, the Twitter stunts, the television talk-show appearances, are all aimed at attracting attention for his causes and investment in his city, they say -- from a program to help ex-offenders set up by the right-wing Manhattan Institute to some $400 million in philanthropic investment in the city and $1 billion in new development. "The mayor is someone who hasn't just talked about fighting for the underprivileged," his spokesman, Griffis, said. "Working to help people has been the the focus of his life."