Regarding impeachment, my question is if it could get through the judiciary committee. It is the House Judiciary Committee that decides whether there are grounds for impeachment. The chair from 1859-1863 was John Hickman, an anti-Lecompton Democrat turned Republican.
The 1858 House Elections resulted in...
Republicans: 116
Democrats: 98
Opposition: 19 >Southern Whigs, later Constitutional Union
Know Nothing: 5
The Committee was chaired by a Republican, but were there enough folks on the committee to get impeachment through it?
And of course, if the secession of the southern states isn't recognized, will deep southern states still be represented in Congress? The Upper South might be testy on the subject.
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If Breckenridge has withdrawn federal forces from the seceded south (SC, GA, MS, AL, FL, LA, TX, and AZ), handed over federal assets, returned state militias to their governors, had the state department accept CSA ambassadors, and presented a treaty to the Senate... what can the north do? Even if impeached, there's the question of whether or not he could be convicted in the Senate. Would deep southern Senators be represented if those states are in rebellion?
And if Breckinridge is impeached, there is no Vice-President at the time. The President Pro Tempore of the Senate (who per the Presidential Succession act of 1792 was to be acting president should the offices of President and Vice President be vacated) was Alabama Senator Benjamin Fitzpatrick, meaning there's a whole other legal/constitutional issue considering Alabama has already declared secession. He didn't withdraw from the Senate until January of 1861. Otherwise it'd be Jesse D Bright (an Indiana Senator and Copperhead who was expelled from the Senate in 1862 for being pro-Confederate), which isn't of much help either. It wasn't until February 1862 that Republican Solomon Foot became President Pro Tempore. Sure, they could impeach and convict Fitzpatrick or Bright as well, but that'd take even more time.
If Breckinridge is impeached and convicted before Foot becomes President Pro Tempore, Speaker William Pennington would be President.
Much would depend on if the senators from seceding states attempted to remain and vote on the treaty. If they did, I would expect the loyalist senators to very quckly reconvene without the secessionists and declare themselves the legitimate Senate/that any session conducted with the participation of senators from seceding states was illegitimate. It's actually an interesting question as to the legality of those senators attempting to remain in the Senate. On the one hand, if you hold that secession is illegal and that states can't leave the Union then there's an argument that their senators remain legitimate members of the Senate. IOTL, that was countered by adopting the position that while the states themselves never left the Union, their governments became illegitiate by attempting to leave and thus the states' seats in the House and Senate became vacant. In a technical sense it would probably depend on when exactly the state in question passed it's secession ordinance and when said ordinance came into effect. If it comes into effect immediately upon passing, then the senators and representatives are out. If, however, the ordinance specified a future date when the legal separation would come into effect, then theoretically the representatives and senators should retain their seats until the date of effective secession. Unless you take the position that announcing the intent to secede has the same effect of delegitimizing the state government as actual secession does? Completely pointless to think about, but fun all the same.
Definitely a fun concept. I'm leaning towards the notion that the House and Senate will utilize the legal rationale that lets them keep the secessionist states out of the legislature the soonest.
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I think the prospect of Breckenridge creating so much political, legal, constitutional, and practical trouble in the union that by the time Lincoln (or Foot or Pennington) enters the White House there might not be much Lincoln could do without extracting a very heavy toll (granted OTL was pretty heavy, but you get my point). Maybe the Union accepts the fait accompli without outright recognizing the Confederacy and Lincoln/Congress pushes for an anti-secession amendment ... resulting in the upper south trying to organize secession. The Upper South also realizes that without the Deep South there's a solid free state/Republican majority, which brings its own issues. Lincoln thus focuses on holding on to the rebelling Upper South (where there are still federal assets and whatnot) while cautiously avoiding the issue of the Deep South. After a couple of years, which is the most I can see this "Planters' Rebellion" lasting, the Confederacy has likely secured some sort of international recognition - making US reaquisiton much more difficult at this point in time.
I think this would be a fun timeline. Legal shenanigans, an independent Confederacy of a sort that isn't the usual cliche, likely a more successful reconstruction in the Upper South, etc.