It's a strange Constitutional quirk that the voters of only a couple states will be lavished with -- and tortured by -- attention from the presidential candidates till November. But it's misleading to think of the election as just a dozen statewide races, because no state is immune from national trends.

As Election Day nears, If President Obama is competitive in Arizona, he's probably winning Florida and destroying Mitt Romney in Wisconsin. Likewise, if there's a close race in Wisconsin, Obama probably has little hope in North Carolina and Ohio. Here's our guide to some of the more interesting of the swing states this year. Right now, Obama's leading in most of them. But a lot can happen in seven months.

State: Arizona
History: The state has voted for a Democratic president only once since Harry S Truman. But the Obama campaign thinks it might be able to make the state competitive, The New York Times' Adam Nagourney reports, and it's seeing how many voters they can register in three months. Obama lost to John McCain by 9 points there in 2008, which isn't huge considering Arizona is McCain's home state.
Why it's competitive now: Changing demographics and a heated immigration debate.
Trends that help Obama: The growing Latino population is not happy with several strict immigration laws passed by the Arizona legislature, The Times reports. Those voters might be energized by the Democratic candidate for an open Senate seat, former U.S. Surgeon General Richard Carmona. Latinos are 30 percent of the state's population.
Trends that help Romney: Latinos are only 19 percent of the voting age population. Latinos typically lag behind others in registering to vote.
Polling trend: The most recent poll, from the right-leaning Rasmussen on March 13, shows Romney beating Obama 51 percent to 40 percent. Real Clear Politics shows Romney leading by an average of 5.4 points. The Associated Press rates it as "leaning Republican."

State: Wisconsin
History: Wisconsin has picked a Democratic president for five elections straight, and Obama won by 14 points in 2008. 
Why it's competitive now: The state has been the center of a huge fight over spending and union power for a year, and many outside groups have poured a ton of money into the recall vote on Gov. Scott Walker. Unions successfully pushed for the recall election after pushed a law to curb union power through the state legislature.
Trends that help Romney: In 2004, Bush lost by only 0.4 percentage points to Kerry, The Washington Post's Chris Cillizza points out. (It was Republicans trying to expand their map that year: "Wisconsin's five major TV markets were all among the top 15 markets targeted by Bush campaign ads," the Milwaukee Journal-Sentinel reports.) And the Obama team has been nervous about Wisconsin for a while -- his first stop after his 2011 State of the Union speech was to a small town there to pitch his economic recovery program.
Trends that help Obama: While Republicans did well in the 2010 elections in the state, Walker is super-polarizing, a poll released earlier this month shows. Plus, "There is no evidence that gubernatorial elections dependably predict a state's performance in presidential elections," Real Clear Politics' David Paul Kuhn says. Obama still has a solid lead in the state.
Polling trend:  The most recent poll, from Rasmussen, showed Obama ahead by 11 points. Real Clear Politics' average shows Obama ahead by 11.8 points. The Associated Press rates it as "leaning Democrat."

State: Virginia
History: Until 2008, no Democrat had won the state since LBJ.
Why it's competitive now: The Post calls this the "swingiest state in the country." 
Trends that help Romney: Republican Gov. Bob McDonnell is popular and considered a possible running mate for Romney.
Trends that help Obama: The growing suburbs around Washington are more supportive to Obama.
Polling trend: Romney led Obama by 5 points in Roanoke College's poll from early April. Two polls in March showed Obama ahead by 8 and 9 points and the president is averaging a 4 point lead. The Associated Press rates it as "up for grabs."

State: New Hampshire
History: The state typically votes for Democrats, though it picked Bush in 2000.
Why it's competitive now: The state had been trending Democratic, but Republicans did well in the 2010 midterm elections, and Republicans have a majority in the state legislature.
Trends that help Romney: Romney won the New Hampshire Republican primary by a large margin in January. He's familiar to voters there, having been governor of a neighboring state and owning a vacation home there, the Los Angeles Times notes. New Hampshire is the only state that picked Kerry in 2004 but Bush in 2000, and the Post says that's likely because of regional loyalties.
Trends that help Obama: Obama's campaign has 30 paid staffers in the state, and has had a campaign office since October, Real Clear Politics' Scott Conroy reports, while Romney's campaign offices are closed.
Polling trend: The most recent poll, from WMUR and the University of New Hampshire, showed Obama up by 10 points in early February. A November poll showed Romney ahead by 3 points. The Associated Press rates it as "up for grabs."

State: North Carolina
History: The state voted Republican for 32 years before picking Obama by a slim margin -- just 14,000 votes.
Why it's competitive now: The financial crisis of 2008 is what put states like North Carolina in reach for Obama, Real Clear Politics' David Paul Kuhn says. It's one of the more likely states for Republicans to take back.
Trends that help Romney: The Democratic governor, Bev Perdue, also won in 2008 by a small margin, and by January of this year, she was deeply unpopular. The New York Times reported that Perdue's decision in January not to run for reelection was cheering for Democrats, who didn't want to be dragged down by her unpopularity, but it's not a good sign that she sunk so far in the first place. Republicans won control the state legislature in 2010.
Trends that help Obama: "The central question is likely to be whether Obama can turn out as many young people in this college-heavy state as he did in 2008," Cillizza writes. "With massive 18-29 turnout, North Carolina looks doable for Obama. Without it, probably not." The Democratic National Convention will be in Charlotte, which Democrats hope will increase their voters' excitement. 
Polling trend: Right-leaning Rasmussen regularly finds Romney ahead by a couple points; left-leaning Public Policy Polling regularly finds Obama ahead by a couple points. The Associated Press rates it as "up for grabs."

State: Nevada
History: The historically Republican state went for Obama in 2008.
Why it's competitive now: The economy is really tough there -- 12.3 percent unemployment in February.
Trends that help Romney: The struggling housing sector and high unemployment rate play into Romney's pitch that he's the economic manager the country needs. Plus, lots of Mormons live in the state.
Trends that help Obama: Lots of Latino voters.
Polling trend: The most recent poll, from left-leaning Public Policy Polling in early April, shows Obama ahead by 8 points, while the polling average at Real Clear Politics shows Obama head by 6.7 points. The Associated Press rates it as "up for grabs."

State: Colorado
History: Bush won the state twice, but Obama won in 2008.
Why it's competitive now: 
Trends that help Romney: The state has a fairly strong Republican history.
Trends that help Obama: The state didn't go as Republican as the rest of the country in 2010, Cillizza points out, as Democrats won the governor's and senate races. Democrats are helped by Latinos, young professionals, and women in the state, the Associated Press says.
Polling trend: Only left-leaning Public Policy Polling has done much polling in the state; it finds Obama consistently ahead 13 points in its April 5 to April 7 poll, a bigger margin than in polls from several months ago. The Associated Press rates it as "up for grabs."

State: Florida
History: It has been a swing state for a long time, picking Obama in 2008, Bush twice, but Clinton only once.
Why it's competitive now: It always is.
Trends that help Romney: Conservative Republicans made huge gains in 2010, with Tea Party-backed Marco Rubio beating former Gov. Charlie Crist in the Republican Senate primary and going on to win the general election. Rubio is considered an appealing potential running mate for Romney. The large Cuban population is conservative, and Latino voters went for Romney in the state's Republican primary. Florida might be tough for Obama, Kuhn writes, because both it and Ohio "shifted to Obama only after the September stock market crash" in 2008.
Trends that help Obama: Latinos are a huge part of the electorate, and non-Cubans are strongly Democratic. It's possible Latinos were turned off by the sharp tone of the debate on immigration during the Republican primary.
Polling trend: Obama was ahead 49 percent to Romney's 42 percent a late March Quinnipiac poll. Obama is averaging a 3.5-point lead. The Associated Press rates it as "up for grabs."

State: Ohio
History: It's been a swing state since 1980.
Why it's competitive now: It was never easy for Obama -- he won the state by 4 points in a landslide election -- and the recession hit the state hard.
Trends that help Romney: Ohio Sen. Rob Portman is considered a top pick for Romney's running mate. The Post says this swing state might be the toughest for Obama to win because voters are older and whiter.
Trends that help Obama: Obama has an advantage in its organization, the Associated Press says.
Polling trend: Obama was up by 8 points in Rasmussen's late March poll, which is about what the polling average shows. The Associated Press rates it as "up for grabs."