Now, Lincoln didn’t do what Dershowitz said he did. He didn’t actually order that the troops be allowed to go “vote for the Republicans” in Indiana; he ordered that the troops be allowed to go vote, period—in the hope and expectation, of course, that they would vote Republican.
But suppose he had allowed them to return home only if they would “vote for the Republicans”? What if he let the soldiers return home only if they took an oath to vote Republican? Or if he gave those willing to take such an oath (but not others) $20 to defray their traveling costs?
In Dershowitz’ view, that could not be an impeachable offense; Lincoln’s “partisan motive” can’t convert something not otherwise illegal into something for which he can constitutionally be removed from office.
If for some reason you are not deeply troubled by that outcome, do bear in mind that there might come a time when a president who does not share your particular political persuasion acts in this way.