I'm not sure if you are still looking for more information on this topic, but I just found a ton while looking through a copy of
The Emergence of Lincoln (Volume 2) by Allen Nevins, so I'd thought I'd post it here since this thread is still fairly recent. First, one illustration in the book features an advertisement sent out by Breckinridge's Democratic National Committee, and the northerners would signed their support for it (and by extension Breckinridge's candidacy) were Isaac I. Stevens of Washington Territory, Augustus Schell of New York, Isaac H. Wright of Massachusetts, John R. Thompson of New Jersey, and Jesse D. Bright of Indiana. I guess Stevens can be added to the list of surprising Breckinridge supporters, as he ultimately became a Union division commander in the Civil War, and died a hero's death trying to preserve the Union at the Battle of Chantilly.
Second, Nevins claimed that of the 10 Democratic senators from the north in this time period, 8 supported the Breckinridge candidacy, going on to list Jesse D. Bright (D-IN), Graham N. Fitch (D-IN), William Bigler (D-PA), and William M. Gwin (D-CA) by name as particularly vocal supporters. From my research, I have found that at the time, there were indeed ten Democratic senators from the north, who were:
- William Bigler (D-PA)
- Jesse D. Bright (D-IN)
- Stephen A. Douglas (D-IL)
- Graham N. Fitch (D-IN)
- William M. Gwin (D-CA)
- Joseph Lane (D-OR)
- Milton Latham (D-CA)
- George E. Pugh (D-OH)
- Henry M. Rice (D-MN)
- John R. Thompson (D-NJ)
Of these men, I have been able to find conclusive evidence for Bright, Fitch, Lane, and Thompson, and have found reasonable evidence of the support of Bigler, Gwin, and Latham. Of course, Douglas and Pugh are the two senators would supported Douglas' candidacy, but I have not been able to find a single scrap of evidence that would lead me to believe that Henry M. Rice, a Minnesota man, supported Breckinridge, so I'm am not 100% certain of the authenticity of the quote, although I am inclined to believe the words of Nevin, considering how distinguished of a historian he was.