Look at the states where it was only Lincoln vs Douglas and the states where it was Lincoln vs. Douglas, Breckenridge, and Bell and the total Democrat vote isn't all that different. It's not like the Bell or Breckenridge supporters were voting for Lincoln when their man wasn't on the ballot.
Take California. Only 3 Ev
The split between Douglas and Breckinridge gave Lincoln California’s three.
California
Margin for Lincoln 0.61%
Lincoln 32.32%
Douglas 31.71%
Breckinridge 28.35%
Bell. 7.60%
Same in Oregon. So California’s ; and Oregon’s 3 make 7
Oregon
Margin for Lincoln1.83%
Lincoln 36.20%
Douglas 27.99%
Breckinridge 34.37%
Bell 1.44%
So the question is what would a unified party have done in the key states of NY, Ohio, Ind, I’ll and Conn.
States Lincoln was just over 50%.
We just need to knock of 21 to get Lincoln below 152 after Org and Cal.
Ohio’s 23 at 51.2% Lincoln would be enough.
Ind 13 and I’ll 11 at 51.09% and 50.69% would also have been enough.
Or NY 35 - a tougher hurdle at 53.7%… but perhaps doable if your party is not split in 2. Douglas did not win many states, but he came in second in a lot.
If the democrats had stood behind Douglas - they could have won.