It is expected that the vice president will be the cheerleader to the party base while the president appears… er…presidential by taking a more centrist role.
Dick Cheney did this for George W. Bush; Joe Biden is the advance guard for Barack Obama. The Obama administration, on the environment, offering a citizenship path to illegal immigrants and now on gay "marriage", is staking out a light position, getting tantalizingly close without giving the advocates all that they want.
As the Republicans will offer less, the calculus is that the particular advocacy group will not stray across the lines. With regard to the gay "marriage" issue, there is an additional reason for caution. In the 2004 elections, it was the issue of gay marriage that played a significant role in George W. Bush's triumph over John Kerry by increasing turnout among social conservatives.
It will be very helpful to the Romney campaign if Obama crosses the line. That will ensure that the conservative base, lukewarm to Romney's candidacy, turns out in force even if Romney lacks endorsement from Rick Santorum and continues to arouse suspicion about his loyalty to conservatism.
In order to prevent that, the surrogates Joe Biden and one William Jefferson Clinton, will be detailed to bear the message to the gays that the administration supports them, but that Obama needs the flexibility of getting re-elected to office to do more. Joe Biden on "Meet the Press" told his interviewers "that marriage should be about being loyal to someone you love, whether that marriage is between a man and a woman, two men or two women."
Education Secretary Arne Duncan, who comes from Barack Obama's Chicago power base, similarly endorsed gay "marriage". Bill Clinton waded into a North Carolina vote proposing a constitutional amendment that would define marriage as between a man and a woman and urged voters to reject the .proposal.
The various lobbies, however, would prefer Obama to be more upfront. But it was Obama who in foreign policy crises coined the expression "leading from behind". It appears that the same strategy is now in place for domestic issues.