President Obama and Mitt Romney both want to win Ohio. Badly.

Leading up to the Nov. 6 battle in the pivotal swing state, a tussle over who is allowed to vote during the 72 hours leading up to Election Day is well underway, involving Obama, Romney and national military groups.

Below is what you need to know about the fight both sides believe is worth waging in a state that could well be decided by a just a few percentage points (or a few thousand votes).

The battle over who can vote during the three days before Nov. 6 is well underway in Ohio. (J.D. Pooley/GETTY IMAGES)

Let’s start in the summer of 2011, when Republican Gov. John Kasich signed a bill that placed new restrictions on early voting and absentee ballots. Democrats, who rely heavily on absentee programs and early votes, fought the law and in September 2011 submitted enough petition signatures to put it on hold and trigger a direct referendum on the law’s future.

Republicans were worried that a direct referendum on the law that most Democratic voters were likely to oppose would galvanize them to flock to the polls in greater numbers come November., aiding Obama and Democrats up and down the ballot. So Kasich signed a bill in May reversing the law.

However, the repeal also reaffirmed a change that ended early voting three days before Election Day. But not everyone is prevented from voting during the final three days leading up to Nov. 6. Members of the military can vote during that window.

Last month, the Obama campaign, the Ohio Democratic Party and the Democratic National Committee fought the provision by filing a lawsuit arguing that allowing military voters to cast ballots during the 72 hours leading up to Election Day but barring other voters from doing so creates inequality and violates constitutional protections.

But the Obama campaign and its Democratic allies were not alone in taking legal action. Fifteen military groups filed to intervene in the lawsuit, opposing the Obama campaign’s position. The groups’ main concern is preserving electoral protections provided to military voters.

That brings us to last Sunday, when the Romney campaign issued a memo saying (in part) of the Kasich-signed law that it “is not only constitutional, but commendable that the Ohio legislature granted military voters and their families this accommodation.” The Romney campaign added that “it is despicable for the Obama campaign to challenge Ohio’s lawful decision.”

Obama senior adviser David Axelrod pushed back on Fox News Sunday, saying: “What that suit is about is whether the rest of Ohio should have that same right. And I think it’s shameful that Governor Romney would hide behind our servicemen and women to try and win a lawsuit to try to deprive other Ohioans of the right to vote.”

What’s next? A District Court judge has set the date for the next hearing, which will take place on Aug. 15. And Democrats are pressing for the rule to be put on hold, even if the case is not ultimately decided before Election Day.

During the final three days of early voting in 2008, Democrats note that 93,000 voters cast ballots in Ohio. And in a close race, numbers on that scale could matter. George W. Bush’s total Ohio margin over Sen. John Kerry (D-Mass.) was about 119,000 votes in 2004.

Extended battles over voter rights, voter fraud and other protocol related to casting ballots often have as much to do with politics as procedure. The back-and-forth in Ohio appears to be an example of politics playing as important a role as anything else.

Restricting the pool of voters during the final 72 hours to military personnel would likely prevent many of the voters the Obama campaign is targeting from voting during the final hours. Both presidential campaigns realize this and are acting according to the self-interest.