With the selection of Paul Ryan as his running mate, Romney has the easy choice behind him.  Now comes the hard choice, the one that could well decide who takes the oath of office in January: what will the Romney campaign do when the left cranks up its slime machine and turns it on Paul Ryan?

For guidance on what not to do, Romney should look to the McCain campaign's handling of the attacks on Sarah Palin.  Remember how mainstream America fell in love with Palin right after McCain selected her, even to the point of causing a run on eyeglass frames that looked like the ones she wore?  But the left started such a vicious smear campaign that her public image went from that of a bold political reformer to a caricature within days.  Instead of seizing on the lies and the hypocrisy of those vicious attacks as an opportunity to expose the true character of the left, the McCain campaign sat back and watched it happen.  The McCain campaign never recovered.

Now it will be Paul Ryan's turn.  Ryan has shown real leadership in his efforts to get the political class to face fiscal reality.  He has come across more like a statesman than a politician, as more concerned with turning this nation away from the approaching fiscal cliff than with political posturing.  He has spoken truth to an American electorate accustomed to having their ears tickled with empty talk of hope and change.  He has exposed the trap of dependency that lies beneath the sweet rhetoric of the community organizer.  The contrast between the personal maturity of the Romney-Ryan team and the adolescent narcissism of the Obama White House could put the Romney campaign back in the game. 

But just give the left's slime machine a week or so, and see what the public image of Paul Ryan will become.  The notorious ad with a Ryan lookalike pushing Granny off a cliff will be clean compared with what is coming over the next few days.  After being called a felon and being accused of causing a woman to die of cancer, Romney has no excuse for expecting decency or honesty from the Obama camp, and he has no excuse for being unprepared for the coming smears against Ryan.

On the surface, the lesson of McCain's failure is that a campaign cannot win from a defensive position.  That much should be obvious by now.  But the deeper challenge for Romney, and for many in the center-right mainstream of American politics, is to figure out how we end up in a defensive position against the left in the first place.  After all, conservatives have won the intellectual battle with the left.  Liberals cannot defend the record of big government in Europe or America.  So how does the political mainstream always end up on defense?   

The disturbing answer is that the left has learned to use our decency against us.  Saul Alinsky and other left-wing theorists urged fellow radicals to use the mainstream's own "book of rules" as a weapon to defeat us.  For mainstream America, that book of rules is part of our Judeo-Christian moral heritage.  Mainstream Americans recognize moral boundaries' limits on our behavior.  In political campaigns, we expect and tolerate spin control to some extent, but we draw the line at lying, viciousness, and deceit as tactics.  We expect our opponents on the left to respect those same moral boundaries, and we are caught flat-footed when they don't.  We try to debate based on the evidence, and evidence is at a disadvantage in the face of lies and distortions.

Republicans who hope that the vicious tactics of the left will alienate voters should remember that Palin briefly energized the McCain campaign before the left started their attacks.  And even now, running with an indefensible record, Obama is comfortably ahead because of his unscrupulous attacks on Romney.  Hardball tactics win in politics. 

The Romney campaign does not have to lie in order to put Obama on the defensive, but Romney and his people do have to be willing to hit hard, hit often, and keep the pressure on.  Obama is accustomed to being on offense, but he is ill-equipped personally or politically to defend himself.  Obama's narcissistic character is well-known, and it could prove to be his Achilles heel if Romney can figure out how to exploit it.  Narcissistic people do not handle criticism well.

It should go without saying Romney has to offer a strong, positive vision for America, but it also should be evident by now that vision is not enough.  Romney has to have personal credibility before his vision can have credibility, and Obama's attacks have succeeded in damaging that credibility with voters whose opinions are shaped by the liberal media or by campaign ads.  If he lets the left get away with their coming smear campaign against his running mate, Romney's credibility, like that of John McCain in 2008, will not recover.

Timothy C. Daughtry, Ph.D, is co-author of Waking The Sleeping Giant: How Mainstream Americans Can Beat Liberals At Their Own Game.