If Mitt Romney is to hit a campaign reset button, this convention speech is his last, best chance.

By now, however, he has drastically constricted his scope to reshape his campaign. He's committed himself to an all-in campaign strategy. With his choice of Paul Ryan as his running mate and his all-but-total endorsement of Paul Ryan's budget plans, Romney has drastically limited his options.

He has pledged himself to a big tax cut for upper-income earners and severe cuts in social spending for almost everybody else. His job now is to convince millions of Americans that their short-term pain will yield long-term gain: more growth, more jobs, a stronger national character.

That's always a tough sell, but as a matter of politics it can be done. (Leave aside for the moment whether as a matter of economics or policy it should be done.) But there is one essential prerequisite for such a sell: the candidate must inspire trust in himself and his fair intentions for all of society.

Mitt Romney comes to this mission with one great asset and one great liability. The asset is a reputation for administrative competence unmatched by any non-incumbent presidential candidate since .. when? George HW Bush in 1988? Lyndon Johnson in 1964? Ike? Hoover?

The liability is a vast personal and social distance from most Americans. Born to wealth and position, inheriting brains and good lucks, blessed with a happy and healthy personal life, he rapidly earned an enormous fortune and launched a political career without so much as a rejected prom invitation to humble or humanize him.

Barack Obama was a pretty lofty individual too, back in 2008, but nothing like Romney. America has elected other rich men to the presidency before. But always, there was something in their story that showed that good fortune had not isolated them utterly: George W. Bush's struggle with alcoholism; George HW Bush's loss of a child to leukemia; Lyndon Johnson's early poverty; John F. Kennedy's wartime heroism; Franklin Roosevelt's polio; Teddy Roosevelt's life of derring-do.

What can Romney tell? And if he can't tell something, the fact that he is proposing a program that will impose great costs on others and huge benefits upon himself becomes a very great political challenge.

- MORE TO COME-