Congress in its vote to authorize military action against Syria for use of gas on its civilian population will have to think about other countries stockpiling and using gas against the United States, or rogue non-State players getting their hands on such outlawed weapons.
At this stage every member of Congress accepts as fact that Assad used gas on a number of occasions. Obama's asking Congress to lend unity by the US will certainly bring forth the UN's analysis which were reported on the Sunday talk shows as Sarin.
Obama is clear that he does not want to be engaged in the Syrian civil war - that is he is uninterested in regime change and helping the rebel forces led by Al Qaeda elements and others hostile to the US. He is not interested in boots on the ground. He is not interested in fly-overs over Syrian air space, and instead relying on destroyer-fired Tomahawks. Obama wants a time-limited strike - at most 2 days. Obama only wants to destroy the delivery systems for gas and the facilities used for making them. He will have to take out some air defense systems first.
Given all that it should be easy for the White House to tailor its request for unity from Congress so that it satisfies them. Congress people have to talk as they do.
This is not about Syria per se. The US does not want another war at this time, but will do militarily what it must to prevent Syria from using gas and Iran from going nuclear in terms of missiles. That's it.