In the 37th Congress, the Republicans would not have had a majority even in the House, let alone the Senate, had there been no secession. (See the tables at
http://en.wikipedia.org/wiki/37th_United_States_Congress and note how the Republican majorities depended on the large number of vacant seats.) There were enough "free state" Democrats sympathetic to the South (Jesse Bright of Indiana is an extreme example) that no legislation hostile to the South could have been passed. Even on the tariff, an issue where some otherwise pro-Southern Democrats from states like Pennsylvania sided with the Republicans, the Morrill Tariff would not have had the votes to pass the Senate had all the southern states stayed in the Union. (This by itself is enough to make nonsense of the notion that the South seceded because of the tariff, even leaving aside the heavy emphasis on slavery in the secession debates and declarations of secession. But there is no use trying to reason with neo-Confederates on this point.)
Why didn't this fact impress the secessionists? Mainly because they were not worried about what *Congress* (at least in the short run) might do. They were worried about what *Lincoln* might do. These fears ranged from the absurd belief that Lincoln was going to deliberately encourage new John Brown raids to the more plausible concerns that Lincoln might use federal patronage to build up an anti-slavery party in the South and that he would have the post office allow the circulation of abolitionist literature in the South. (There was also the fear that the very fact of Lincoln's election could encourage slave revolts, whether Lincoln wanted them or not. John Slidell said in his farewell adsress to the Senate that Lincoln's inauguration would be viewed by the slaves as "the day of their emancipation." John Bell, hardly a fire-eater, wrote a public letter in which he said the "simple announcement to the public that a great party at the North, opposed to slavery, has succeeded in electing its candidate to the Presidency, disguise it as we may, is well calculated to raise expectations among slaves, and might lead to servile insurrection in the Southern States."
http://www.nytimes.com/1860/12/12/n...rpose-tendency-republican.html?pagewanted=all)
And of course with the Republicans controlling the executive branch, it was irrelevant that Congress did not have the votes to impose a Wilmot Proviso on the territories; as Kansas had demonstrated, no new slave state could be formed out of the territories without active executive encouragement. In fact, during the "secession winter" Republicans readily agreed to the organization of new territories without any restrictions on slavery precisely because they knew such restrictions were now superfluous.
All in all, attempts to portray Lincoln's election as an *immediate* threat to slavery were unconvincing and even paranoid. OTOH, that a Republican victory was a long-term threat to slavery was more plausible. If the South failed to secede, the Republican party would become a permanent factor in national politics--after all, the big argument against it, namely that its victory would lead to disunion, would have been proven untrue. Even Democrats in free states would compete with Republicans in anti-southern sentiment; already, there was a Douglasite "semi-free-soil" wing of the Democratic Party which *in practice* was as opposed to slavery expansion as the Republicans. No more slave states would be admitted to the Union, while one free state after another would be. I don't think that secessionists were wrong to think that in the long run this would be fatal to slavery, even if they were vague in saying exactly *how.* The point is, that if the slave South was now faced with inevitable decline of its influence in the Union, it had to either accept the eventual end of slavery or get out of the Union *now.* Lincoln's election represented a turning point which made secession plausible in a way it would never be in the future. The future victories of antislavery might be so gradual, and the South might be so demoralized, that it would never resist them.