Mitt Romney’s campaign bus neared the end of a five-day tour of six potential battleground states Tuesday. Most could be competitive in November, but none looms larger on the Electoral College chessboard than the place where he stumped on Sunday: Ohio.

Barack Obama won all of the states on Romney’s itinerary in 2008. One, Wisconsin, hasn’t gone Republican since 1984. Two others, Michigan and Pennsylvania, have been in the Democrats’ column for five consecutive elections. Two others, Iowa and New Hampshire, have gone back and forth over the past 10 years.

But of the six, Ohio remains the bellwether. It has been at the center of presidential politics in the past three elections and is once again destined to play a pivotal role this year. If Obama can win Ohio, then he almost certainly will be re-elected. No Republican has won the White House without Ohio. If Romney is hoping to convert some of the other, bluer battlegrounds, his progress in Ohio could be an indicator of how things could break elsewhere in the industrial heartland.

Romney’s bus tour is a way to test the viability of some of these more difficult states. A few months ago, Michigan appeared to be leaning strongly toward Obama. That was in the middle of the Republican nomination contest, when Romney scrambled to win the primary in Michigan, his home state, against a surging but ultimately sagging Rick Santorum.

Wisconsin also looked challenging as Romney sought, successfully, to finish off Santorum in a presidential primary that was overshadowed by a roiling recall campaign against Republican Gov. Scott Walker that deeply divided the state. At the time, some Democrats thought Wisconsin would be secure for President Obama no matter the outcome of the recall effort.

Democrats’ confidence was based on the fact that Obama carried Michigan and Wisconsin by double-digits four years ago. Recent polls show both states a lot tighter than that, although by how much isn’t quite clear.

Among the other states on Romney’s tour this week, Iowa is a true battleground. It was the state that launched Obama toward the presidency in early 2008, but Democrats acknowledge that the pounding he took during the Republican nomination battle has left him weakened. Symbolically, Iowa will be hugely important to the president, but it’s clear that he’ll have to fight to hold on to the state.

If Obama hopes to get a boost from Iowa’s proximity to his home state of Illinois, Romney hopes that his tenure as governor of Massachusetts will make him truly competitive in New Hampshire. That’s where he began his bus tour on Friday, at the same picturesque farm where he formally started his campaign a year ago.

Of the six states on Romney’s tour, Pennsylvania remains the most difficult for him. Republicans had a good year there in 2010, and GOP strategists think Romney can do better than some past nominees in the Philadelphia suburbs. He has yet to prove that.

That brings the focus back to Ohio, a state whose 18 electoral votes are a crucial part of the calculus for both campaigns. Obama’s strategy always has been to avoid being trapped in a campaign whose outcome depends on Ohio and Florida. He has other routes to victory, but Florida looks more challenging for him this year. Investing more in Ohio and less in Florida could make sense, although the president’s campaign is likely to spend heavily in both.

Ohio aligns closely to the nation in presidential races, with a slight tilt to the Republicans. In the past three elections, the Democratic nominees have been about two percentage points short of their national percentage twice (2000 and 2008) and less than one point above once (2004).

Ohio also offers a laboratory for a debate that will play out this week in several other states on Romney’s tour, and that is how voters should assess the economy. Ohio, Wisconsin and Michigan are seeing signs of economic improvement.

Ohio’s geography, located in the center of a group of potential battlegrounds, and its history through recent campaigns, gives it once again the special prominence it has had before.