In particular, Lincoln stated the objection this way: that the president, "who is sworn to 'take care that the laws be faithfully executed,' should not himself violate them."
But when he addressed Congress on July 4, 1861, Lincoln defended his decision on two grounds: that it was essential to preserving the Union and that it was constitutional.
"Are all the laws, but one, to go unexecuted, and the government itself go to pieces, lest that one be violated?" Lincoln asked. "Even in such a case, would not the official oath be broken, if the government should be overthrown, when it was believed that disregarding the single law, would tend to preserve it?"
And Lincoln cited the provision in the Constitution - Article I, Section 9, clause 2 - that authorized his action: "The Privilege of the Writ of Habeas Corpus shall not be suspended, unless when in Cases of Rebellion or Invasion the public Safety may require it.''