WASHINGTON — Republicans learned the lesson the hard way: In the new arithmetic of presidential politics, counting on Election Day votes doesn’t always add up to victory.
In 2008, John McCain won the majority of votes cast at the polls that day in the crucial states of North Carolina, Florida, Iowa, and Colorado, but his victory margin was wiped out by support for Barack Obama among the millions of voters who cast ballots early. The result: McCain netted not a single electoral vote in those four key states.
This year, early voting will be even more essential, with about 40 percent of the ballots cast before Nov. 6 — up from about 31 percent in 2008. And Republicans are competing vigorously, more so than ever before, even as the party has tried to limit early voting in some areas, most notably in Ohio, where Obama could benefit.
Trying to match vaunted Democratic field operations in tossup states, Republicans are mining troves of data to identify possible supporters and then help them vote early: offering guidance on applying for and mailing absentee ballots, reminding them of deadlines, and offering rides to early balloting locations.
As of Wednesday, at least 6.5 million Americans had voted.
“Our mission is to cut into the gap from 2008,’’ said Kirsten Kukowski, a spokeswoman for the Republican National Committee. “Our focus right now is in getting people who wouldn’t necessarily come out to vote, so on Election Day we can focus on getting out our base.”
Users of absentee ballots have traditionally favored Republicans, who are more likely to be conservative, older, and white. As other types of early voting were expanded by many states in 2008, the Obama campaign made it a centerpiece of its strategy to reach out to sporadic voters in low-income neighborhoods, minority enclaves, and college campuses — with the goal of getting them to the polls early.
“The Republicans ignored it to their detriment four years ago, and the Romney campaign can’t afford to ignore it,” said Michael P. McDonald, an associate professor of public and international affairs at George Mason University. McDonald has been tracking the early votes.
In particular, Republicans are targeting rural voters, a contingent that analysts say will need to show up strong for Romney to have a chance of winning the short list of tossup states. For months, the Romney campaign has been urging rural residents to vote by mail, saving them the trouble of traveling the long distances they may face to vote on Election Day.
The exhortations from the candidates are almost daily.
“Please don’t forget early voting,’’ Representative Paul Ryan, the GOP vice presidential nominee, told those at a campaign event in Cleveland last week. “Ohioans have a unique responsibility. You are the battleground of battleground states.”
While ground forces face off in the trenches, some state Republican parties have taken a parallel track: Limit early voting, especially in a state like Ohio where early votes could tip in Obama’s favor. Last week the US Supreme Court let stand a lower court ruling that sided with Ohio Democrats who challenged a GOP-sponsored measure to cut off voting three days before Election Day for most voters. In 2008, observers said, Democrats made up the majority of voters in those final days.
Although the early ballots will not be counted until Election Day, analyses of those who have already voted in tossup states hint at the challenges Republicans face.
■ Since Ohio opened early voting Oct. 2, more than 812,000 residents have cast ballots, or about 14 percent of the 5.8 million tallied in the 2008 presidential election. One survey of some of these voters taken by the Wall Street Journal/NBC gave Obama a sizable lead, 63 percent to 27 percent. Predictably, the Romney campaign downplayed the poll, saying that it captured only a few days of voting.
No Republican has won the White House without winning Ohio.
■ In Iowa, about a fourth of likely voters have done so, with 47 percent of those 376,000 ballots cast by Democrats and about a third by Republicans, according to a tally compiled by George Mason’s elections project. Romney is doing slightly better than McCain did in early voting in 2008, but the trend so far is not favorable, McDonald said. “It will be more and more difficult for Romney to make up the difference.”
■ In Nevada, registered Democrats have cast nearly half of the 215,000 ballots since voting started last week, with Republicans accounting for 37 percent, and independents and other voters 16 percent.
■ Until in-person voting began last Thursday in North Carolina, Republicans had the edge among voters who had already filed absentee ballots. But that advantage vanished when an estimated 150,000 voters turned out on the first day of in-person voting, most of them apparently Democrats. By Wednesday, about 817,000 people had voted, representing 19 percent of the 4.4 million who voted in 2008. Democrats were showing up in larger numbers than Republicans, 51 percent to 30 percent, according to the George Mason breakdown.
Massachusetts does not have early voting but allows absentee voting with an excuse.
Among other tossup states, in-person early voting started in Colorado on Monday and will begin in Florida on Friday. In the Sunshine State, more than 926,000 people have already turned in absentee ballots, 45 percent of them from registered Republicans and 39 percent from Democrats.
There are lessons to be learned by the Obama campaign. In 2004, when John Kerry sought the White House, the senator from Massachusetts led Bush 52 percent to 41 percent in early voting in Iowa, polls said then. The cushion was not enough to cover the shortfall on Election Day, with President Bush squeezing out a win, 49.9 percent to 49.2 percent.
At Broward College in Florida on Monday, Michelle Obama made her pitch, urging supporters to get the task out of the way, just in case “you wake up on Election Day and you’re sick, the car broke down, there’s no babysitter,” she said. “You don’t have to chance it.”
With the race expected to be decided by the slimmest of margins, neither campaign is leaving anything to chance.
“If you can get them to the polls early, that’s a guaranteed vote,” said John Hudak, a fellow in governance studies at the Brookings Institution.
“If they wait until Election Day,” he said, “their vote is still up in the air.”Bobby Caina Calvan can be reached at bobby.calvan@
globe.com. Follow him on twitter @GlobeCalvan.