The TPP is horrible. Among other things, it substantially limits the ability of the government to regulate Wall Street. It makes an international agreement supreme over U.S. law.
President Obama and the Republicans are trying to "fast-track" the deal, which means its contents will get no public scrutiny, no review by congressional committees. We won't know all of the corporate welfare the deal includes until it is done.
I don't know what possessed President Obama to do this. It is undemocratic. It is dangerous. Certainly all Democrats must oppose this bill. It we do, then even with Obama's support, we can make it an issue against the Republicans in the next election. And if any Democrat votes for it, that can be used against that member of Congress in the Democratic primary.
Robert Reich has a great two-minute video that is very convincing.
See it here.