Narendra Modi's advance toward Delhi had been near effortless. Rivals within his own party fell easily by the wayside, investigations into the riots ran into legal dead-ends, even as the ruling government toppled headlong into a self-engineered free fall. That's until a hot-headed, insufferably stubborn and unpredictable former bureaucrat came along, and toppled the well-oiled BJP apple-cart.

No, Arvind Kejriwal has little or no chance of becoming India's next prime minister. But he is perfectly positioned to deny that prize to its leading contender. And here are ? reasons why.

One, street fighter Kejriwal pulls no punches. It has been absurdly easy for Modi to take on a tongue-tied "shahzada" and his band of loose-tongued supporters. Where Rahul does himself in by saying too little -- for example, his infamous "people died" response to the Gujarat riots -- his party men compound the damage by saying too much, as in calling Modi a "chaiwallah." It is easy to look like a master communicator when your opposition is a bunch of PR-challenged bumblers.

Kejriwal, however, will match Modi's rude rhetoric, inch for inch. He will name names, and shout out every tawdry allegation -- from Adani/Ambani to Godhra to Snoop-gate -- with merry abandon. Where the Congress party is the proverbial kettle, unable to call anyone "black", AAP is perfectly positioned to play the stainless steel bartan, as Yogendra Yadav has made amply clear :

"We have decided to take on the BJP and Modi frontally. To begin with by consistently asking questions that have not been asked in the public domain. Perhaps the Congress doesn't feel it is in a position to ask these questions. We will raise certain issues. We will use the social media platform which is a big platform now. We will raise these issues and go to the streets demanding answers. Modi is after all seeking to be PM of India so the questions will have to be raised everywhere across India."

Attacking Modi's integrity fits perfectly into AAP's modus operandi and image. And unless damaging skeletons lurk in Kejriwal's own background, the Gujarat Chief Minister will unlikely be able to return the many "compliments" that will be coming his way.

Two, Kejriwal is no shahzada. A Rahul-Modi fight is a dream come true for the BJP -- and perhaps one reason why they decided to place all their eggs in NaMo basket. Who better than a self-made chaiwallah to take on the princeling, to damn his dynastic privilege by merely the fact of his humble background. The last thing the BJP needs is some aam aadmi to come along and ruin this unfolding morality play.

With Kejriwal, there can be no snickering about Mummyji or awkward Hindi or Harvard degree. It is easier to mock staged dinners with Dalits than a chief minister who thinks little of spending a cold winter night on the pavement -- a feat Modi will find difficult to match. Sure, Modi's professional class constituency may not be quite as entranced by sleepovers on the streets, but as Charles Correa writes, "Remember that over 60 per cent of Mumbai’s inhabitants, by government count, are squatters. Sleeping on a pavement may not seem as foreign to them as it might to you or me… It’s an image to which the silent majority of our urban citizens might relate—instantly and without much effort."

If Kejriwal plays his cards right, Narendra Modi may well find himself on the other end of the stick, defending his well-established image as a powerful career politician against an ordinary man of the streets.

Three, Kejriwal offers the option of the not-Modi/BJP vote. Contrary to what Modi supporters insist, many who are reluctant to vote for him or the BJP are not necessarily Congress supporters. Nor are they just anti-free market intellectuals or victims of vote bank politics. There are vast swathes of traditional Congress constituencies who have lost faith in the grand old party, but are uncomfortable with the BJP brand of authoritarian/majoritarian politics. The urban educated class may have lost some faith in him, but his resignation has raised his stock with many others who feel disempowered and unrepresented.

Many of these people would have set aside their doubts to vote for BJP because, as Manu Joseph put it, "Modi to millions of Indians, appears to be a more useful leader than the sofa-cum-bed leadership of Sonia Gandhi and Rahul Gandhi." But AAP now gives them a reason not to. Or at least that's the party strategy, as Yadav lays it out:

Today, when the Congress is being marginalized, there are substantial sections of Indians who would not want to go for a substitute. This section includes idealistic youth who would not wish to go back to one more party that does business as usual. The poor and disadvantaged simply do not relate to the BJP as a party. Also the Dalits, Adivasis and Muslims are looking for an alternative as they do not have reasons to trust the BJP. In such a scenario the Aam Aadmi Party sees itself as providing that alternative.

In the months to come, AAP will attempt to deepen and underline lingering doubts and fears about the BJP among the fence-sitters, hence its full-on frontal assault on Modi. In its latest avatar, AAP is the "you don't have to vote for Modi" party. And there are sufficient numbers who don't want to for Modi to worry.

Writing in the Indian Express, Pratap Bhanu Mehta writes of the two countervailing forces in the psyche of the Indian electorate. On one hand is the fear of plutocracy: the sense that "the rich control politics, they control the banks, and then spray pepper into peoples’ eyes." On the other is the fear of paralysis: "a sense of the country drifting like a ship in the night without direction, without a captain." He argues that Kejriwal's confrontational strategy in Delhi may increase "a yearning for predictability and order, even if a bit tainted," concluding that Modi "will gain most from this fracas."

And so he will among those who have seen the greatest gains from economic stability, however tainted, and therefore yearn for a leader who will match, even outstrip, the giddy days of UPA 1. But there are others who have not gained -- or at least not enough -- from this vaunted stability, or have paid a higher price for it, and are willing to bet on a high-stakes gambler who promises real change.

The outcome of the 2014 elections will be determined not by personalities but the murky electoral math of coalitions. The election itself, however, is now a true contest, a real clash of the titans. Kejriwal will not win, but he will draw blood, may be enough to decisively weaken Modi. And that is just fine with him. Kejriwal's biggest advantage over his rival is simple: Unlike Modi, he has nothing to lose.