Tuesday night, arguably the most popular campaigner of this presidential race will have her say, when Michelle Obama takes the stage to make the case for her husband’s reelection.

And while her role as candidate’s wife remains more or less the typical one—humanize the hubby—expect the contrast between Mobama’s convention appearance and Ann Romney’s to be dramatic in a number of ways.

Obviously, the two women are worlds apart with regard to their life paths and their politics. In many ways, Michelle has been the model of the 21st-century, struggling-to-balance-it-all woman. By contrast, Ann, with her golden sweetness and refinement and traditional stay-at-home role, makes Laura Bush look like a bra-burner.

This is not to suggest Ann is cloying or offputtingly perfect. By dint of her health struggles] (not to mention raising those five boys), she conveys a quiet strength and relatable quality. At the same time, however, she radiates an almost girlish vulnerability. During her convention address, she had a tendency to giggle and look overwhelmed by the outpouring of affection from the crowd. And while her delivery was solid, she was clearly out of her element—which was, of course, part of the appeal. You wanted to wrap an arm around her shoulder and gently escort Mrs. R. through what Team Romney had (shrewdly) stressed was her first-ever dance with a big bad Teleprompter.

A similar aura pervades Ann’s media interviews. Despite being a highly accomplished veteran of the campaign trail, she comes across as somehow too gentle for this whole dirty business of politicking. Oh, she knows how the game is played, and occasionally even enjoys the rough and tumble. (Let us recall how, at a closed-door fundraiser in April, Ann referred to Hilary Rosen’s unfortunate swipe at her as “my early birthday present” because of all the blowback it caused. ) Yet she manages to seem as though she’s removed from all that.

By contrast, neither fans nor foes think of Michelle Obama as particularly gentle or vulnerable or removed. The woman is a force of nature: tough, spirited, saucy, straight-talking, fierce. Among observers, she does not engender protective feelings so much as awe—and, among detractors, anxiety.

If Ann Romney’s convention speech was, as she adorably put it, all about “love,” Michelle Obama’s will be all about passion. Not hers for Barack (or vice versa), but his for this country, all of its people, and the values about which so many people got so amped up in 2008.

This time, Michelle doesn’t need to introduce or explain her husband so much as she needs to remind people of why they loved him in the first place—and, more vitally, why they should stick with him. To this end, she needs to do three things: 1. Walk the crowd back through both her and Barack’s modest beginnings. 2. Run through the highlights of what her husband has thus far accomplished despite all the partisan opposition. 3. Draw stark distinctions between her husband’s priorities and those of the other guys.

All of these elements have been on display in the first lady’s stump speech of late. Just a couple of weeks ago, she made a campaign stop at Bradley Tech High School in Milwaukee, where she hit all of the above themes. Hard.

A couple of money lines from that speech to listen for on Tuesday night:

“This election is a choice.” 

This, more or less, is the president’s central campaign message. The Romney-Ryan ticket (like most challengers) want Americans to treat this race as a referendum on the incumbent. Team Obama, in turn, must stress that voters are in fact choosing between two competing candidates with two competing visions.

Or, as Michelle told the Milwaukee crowd: “In the end it all boils down to who you are and what you stand for.”

Ouch. This is a particularly pointed barb to level at Mitt Romney, whose central failing thus far has been his inability to reassure voters that he is anything other than an awkward, out-of-touch guy who stands for little. (Or, more harshly, who stands for little apart from the interests of other very rich people just like him.) Indeed, expect this to be one of the dominant themes of the entire Democratic convention.

Ann won plaudits for softening up her husband’s image. Michelle knows her job is different—and trickier. She needs to fire up voters on her husband’s behalf without going too far and enabling critics to stereotype her (as they did in ‘08) as the Angry Black Woman. 

Fortunately for Mrs. O, she’s spent the past four years learning to thread that needle.