By Mujtaba Rahman and Willis Sparks 

It's not easy to unseat an incumbent U.S. president. In 80 years, only Ronald Reagan (1980) and Bill Clinton (1992) have done it, and while it's too soon to say how close the current race will be, it's clear that Republican challenger Mitt Romney lacks the ease, warmth, and charisma of these two remarkable political talents. Mitt Romney needs a boost. 

Given that U.S. economic data points to a recovery that doesn't appear on the verge of either a surge or a sharp reversal, the most obvious risk for the Obama campaign is a substantial market meltdown in Europe in September or October, one that sends Wall Street into a tailspin and generates consumer and investor fears that the U.S. economy is again headed in the wrong direction. Though Europe's markets may well endure a rough ride this fall, a substantial meltdown is unlikely. But given the likely timing, it's a risk that both candidates are thinking about.

For now, President Obama is winning. Despite historically high unemployment and continued consumer anxiety over the state and direction of the U.S. economy, current polling in the states likeliest to decide the election gives President Obama several credible paths to victory. Judging by their spending habits, the two campaigns and their allies believe the race will be decided in Ohio, Florida, Virginia, North Carolina, Colorado, Nevada, Iowa, New Hampshire, and perhaps Wisconsin. Romney will have to win six or seven of these nine states to earn the 270 electoral votes needed to win, and only in North Carolina does he enter the fall with a clear lead.

Given the country's intense current partisanship, party faithful on both sides are likely to vote in large numbers this November, and the outcome of a close election will depend on the choices of genuinely independent voters, a group that represents about 10 percent of those likely to cast ballots. This is, by definition, the least ideological segment of the U.S. electorate, and these voters are most likely to vote based on perception of the president's competence and on confidence in his leadership rather than on fidelity to a defined set of social and political values. That is why, though he remains the favorite, President Obama remains vulnerable to external events over which he has little direct influence.

Which brings us to Europe. The autumn to-do list facing Eurozone leaders is daunting. They must agree on another broadly unpopular bailout for Greece, manage volatility in Spain and Italy, and create a credible new Eurozone-wide bank supervisor to make future crises both less likely and less costly to manage when they occur. They must also agree on a concrete and detailed plan to address the Eurozone's original design flaws, and they must do all this with jittery investors watching their every move.

Why is the fall likely to be an especially volatile time? Greece will return to center stage in September, given the additional financing needs that have been created by poor growth, a bureaucracy slow to implement promised reforms, and the possible extension of fiscal targets. Creditor countries are not eager to help, and they may demand that Greece's government enact reforms before help is provided, leaving Greek officials caught between an increasingly angry public and the expectations of those who can help. In the meanwhile, warnings from creditors -- designed to keep pressure on Greek officials -- will keep markets on edge. Also in September, we can expect a ruling from Germany's constitutional court on whether the Eurozone's proposed bail-out fund-the center piece of Eurozone leaders' response to the crisis-is legal.

Then there is the trouble in Spain. Current yields on Spanish debt reflect market expectation that the European Central Bank (ECB) stands ready to backstop Spain. That's an overly optimistic assessment, because ECB President Mario Draghi has detailed the pre-conditions for the bank's help, and those conditions have not yet been met. To satisfy Draghi's requests, the Spanish government must first make use of sovereign-backed bailout mechanisms in conjunction with a reform program. But unless and until market pressures force him to, Spanish Premier Mariano Rajoy is unlikely to ask for help, increasing the risk of market turmoil.

Also this fall, the European Commission is due to propose plans for a single pan-Eurozone supervisor, part of a deal agreed by Germany in June that will pave the way for direct recapitalization of Spanish banks instead of an indirect path through Spain's government. But many controversial issues remain, not least the separation of national from EU-wide supervisory powers and a decision on whether all Eurozone banks will be covered or whether some countries, especially France and Germany, can negotiate exemptions. Without this measure of reassurance, lenders may again drive rates dangerously high.

Even if agreements on all these issues are reached with a minimum of friction, there is no guarantee that the creation of a supervisor will be enough to shore up vulnerable banks, provoking new anxieties and more market pressure on the bloc's weaker economies.

Will all these potential problems generate enough fear in September and October to send European, and then U.S. markets, reeling? Probably not. But the risk is real, and the timing is ominous. You can be sure that the Obama and Romney campaigns will be watching closely.

Mujtaba Rahman is an analyst in Eurasia Group's Europe practice. Willis Sparks is an analyst in the firm's Global Macro and United States practices.