Let us consider the reasons, pro and con.
In favor:
1) He is one.
2) He called himself one.
3) There is no evidence of an ideological shift.
4) He supports the Sandinistas who are socialist and admit it.
5) He supports ACORN, Occupy, the New Party, and the Working Families Party, which are socialist and don’t admit it.
6) Honesty is the best policy.
7) American democracy works best when people know what they’re voting for.
Against:
1) Calling him socialist may cost him the election: bad for Democrats.
2) Calling him socialist may not cost him the election: even worse for Democrats.
3) Calling people socialist seems rude, especially when they are.
4) Respectable Republicans would rather not seem rude.
5) Obama parallels uncomfortably close.
6) Honesty is not the best policy.
7) American democracy works best when people pretend not to know what they’re voting for.
Conclusion: Too risky. Best not.