It is the function of the Supreme Court to make a determination of constitutionality. That is not the job of the President
I have followed the news on this issue and have learned that it is believed to be unconstitutional for several reasons, The most common being violation of the free speech ammendment.
When stripped of the unconstitutional sections, the remaining portions of the bill double hard money. Republicans win in hard money contributions.
In the end, after the SCOTUS moves, we win. Therefore, it's ok to sign the bill.
Cannot the President just let the bill sit on his desk for a few weeks. It then becomes law without his signature. He can say that he respects the fact that so many senators and congressman have voted to pass this bill, obviously they have listened to their constituents < sarcasm > but due to his concerns over the legality of certain parts of it, he will not his a stamp on it. Then point out how it will be examined through the courts. I support full disclosure of how much and who is contributing.