- Jefferson was the biggest proponent, AFAIK. He'd long held the view that trade sanctions were America's best foreign policy tool. I think he'd advocated something like the Embargo act as early as the Adams administration as an alternative to the Quasi-War.
- Federalists, and New Englanders in general, were strongly opposed to it, due to a combination of Anglophilia and Federalist New England's economic dependence on shipping.
- Jefferson's Treasury Secretary, Albert Gallatin, was opposed to it because he (correctly) believed it would be unenforceable, unpopular, ineffective, and economically devastating.
- There was a pretty strong streak of "something must be done" going on at the time with regards to relations with Britain. Without the Embargo Act, the debate would turn to whether or not to declare war. I don't know how much support there would have been in Congress for a declaration of war, or if Jefferson would try to veto it if it passed. I'm also not sure how split the Democratic-Republican party would be over the war issue if it were at the forefront of the public debate, and I wonder if there'd be enough division to create a party split in the 1808 election campaigns.
The Embargo act also gave a temporary boost to the declining fortunes of the Federalist party, which still lost both Presidential and Congressional elections badly in 1808 but much less badly than they could have (the Federalist House delegation doubled in the 1808 elections from 26 seats (18% of the chamber) to 50 (35% of the chamber). Without the Embargo act as an issue, the Federalist collapse might be accelerated, especially if Federalist leaders decided to back their preferred Democratic-Republican candidate in a party-split scenario rather than running a candidate of their own.