I have never understood the idea of a mandate.
Why 55% should give an elected official significantly greater legitimacy than 50.1% is beyond me.
The greater the margin of victory, the more it signals a public sentiment in favor of the winner and his agenda. With a large enough margin of victory, even minority members of Congress will go along with the President in order to protect their own seats against public backlash back home (at least those who don't have "safe" seats).