“Ambitious men never believe others aren’t the same.”

Source: Time and Again (1970), Chapter 8 (p. 108)

Sometimes ascribed to Robert Browning, this is in fact a misquotation from Robert Burton's The Anatomy of Melancholy (1621): "They [i.e. ambitious men] may not cease, but as a dog in a wheel, a bird in a cage, or a squirrel in a chain, so Budaeus compares them; they climb and climb still, with much labour, but never make an end, never at the top".
Misattributed

Section 2, member 3, subsection 11, Concupiscible Appetite, as Desires, Ambition, Causes.
The Anatomy of Melancholy (1621), Part I

1790s, First Principles of Government (1795)
Context: It is never to be expected in a revolution that every man is to change his opinion at the same moment. There never yet was any truth or any principle so irresistibly obvious that all men believed it at once. Time and reason must cooperate with each other to the final establishment of any principle; and therefore those who may happen to be first convinced have not a right to persecute others, on whom conviction operates more slowly. The moral principle of revolutions is to instruct, not to destroy.

As quoted in "Still breaking the mould" by Gordon Burn in The Guardian (11 October 2005)

The Dangers of American Liberty (1805), in [Ames, Fisher, and Seth Ames, Works of Fisher Ames: with a selection from his speeches and correspondence, 1854, Little, Brown, 349, Boston, http://books.google.com/books?id=fjoOAAAAIAAJ&pg=PA349&vq=known+propensity]

Source: Novels, Anonymous (2013), Chapter 1

XVIII, p. 484
1810s, Letters to John Taylor (1814)

Regarding the resolution of the Nullification Crisis, in a letter to Andrew I. Crawford (1 May 1833).
1830s
Context: Hemans gallows ought to be the fate of all such ambitious men who would involve their country in civil wars, and all the evils in its train that they might reign & ride on its whirlwinds & direct the Storm — The free people of these United States have spoken, and consigned these wicked demagogues to their proper doom.