BEAVER (Pennsylvania) - THERE are battleground states in the US presidential election, and then there are Pennsylvania and Michigan, looming larger than all others because they offer such a rich opportunity for Republican John McCain and potential peril for Democrat Barack Obama.

What worries Mr Obama, and gives Mr McCain hope, is that both states have hundreds of thousands of white, mostly working-class Democrats who seem wary of Mr Obama. In the Pennsylvania primary they gave Mrs Hillary Rodham Clinton a big win over Mr Obama, and now Mr McCain is wooing them hard.

'I need Pennsylvania,' Mr Obama told a crowd of several thousand at an outdoor rally in Beaver on Friday night. 'I need Beaver, Pennsylvania. I need you to stand up beside me and say now is the time to bring about change in America.'

In the crowd, Ms Kim Stelmach of Pittsburgh cheered, and fretted a bit. Despite having young twins at home, she finds time to volunteer for Mr Obama, and is well aware that Pennsylvania is a must-win state.

'I'm extremely nervous,' she said of Mr Obama's standing with white working-class Democrats. 'That's why I'm volunteering.'

Pennsylvania and Michigan have thousands of white working-class voters who call themselves Democrats but sometimes vote Republican.

They are being told by Mr Obama's people that their personal and political interests should trump any qualms about casting a presidential vote for a black man in his first Senate term.

If Mr McCain carries either state, he could lose several states that President George W. Bush won and still claim the White House. For Mr Obama, a loss in either would put him in a deep hole, forcing him to win numerous states that have voted Republican in recent elections to have any hope of prevailing on Nov 4.

It's no coincidence that Mr Obama and his running mate, Delaware Sen Joe Biden, made Pennsylvania their first stop on Friday after leaving their party's convention in Denver, with several Michigan stops scheduled for Sunday and Monday. Mr McCain and his new running mate, Alaska Gov Sarah Palin, held a rally on Saturday in Washington, Pennsylvania, and they have promised to visit both states repeatedly.

There are plenty of other vigorously contested states: Florida, Ohio, Missouri, Colorado and Virginia, to name a few. But Michigan and Pennsylvania are different.

Democrats John Kerry and Al Gore carried both in their losing campaigns in 2000 and 2004. So they form an almost must-win minimum for Mr Obama. He would start with the base those two men had, and then try to pick up enough Bush-carried states to put him over the top.

Polls show Mr Obama slightly ahead in both Michigan and Pennsylvania, but Mr McCain may be within striking distance.

To be elected, Mr Obama must win 18 more electoral votes than Mr Kerry did four years ago. If he loses Pennsylvania, his deficit jumps to 39 electoral votes. If he loses Michigan instead, the gap is 35.

Those are big numbers, because the Bush-won states that look most promising for Mr Obama tend to be small, with few electoral votes.

The possibilities and math can get complicated.

Suppose Mr Obama carried every state that Mr Kerry did, including Michigan and Pennsylvania, and then added Mr Bush's states of Iowa, New Mexico and Nevada, all prime targets this year. He would still lose to Mr McCain.

But if he grabbed one more state that Mr Kerry lost - Colorado, for instance - then he would be president.

Mr McCain, on the other hand, could lose Ohio or Florida (both won by Mr Bush in 2000 and 2004) and essentially offset it with a win in Michigan or Pennsylvania. Swapping Florida for Michigan would cost McCain 10 net electoral votes, but he wouldd still win the election if all other states followed 2004 results.

Mr Obama's challenge becomes far greater if he loses either Pennsylvania or Michigan. Even if he won all the other Kerry states from 2004 and added the Bush states of New Mexico, Nevada, Iowa and Virginia - a state with 13 electoral votes that Democrats have not won in decades - he would lose the election.

Michigan and Pennsylvania have been hit hard by the long decline in heavy manufacturing jobs, especially in the steel and auto industries. Mr Obama tells workers (and unemployed people) that Republicans have abandoned them. He promises to invest in technologies that will create jobs, and to cut middle-class taxes to help families pay their bills.

Mr McCain also promises to bring more jobs to the heartland. He places more emphasis on across-the-board tax cuts and greater flexibility in finding health insurance.

Mr Obama did not campaign in Michigan during the primary because a Democratic Party dispute essentially negated the state's primary election. He needs to make up for lost time, says Michigan Gov Jennifer Granholm.

He needs to 'come, come, come to Michigan,' she said. The state's wary voters, she said, need 'to feel him, see him, touch him.' Mr McCain, too, would love nothing better than to win them over, which could give him a victory in Michigan and a huge step toward the White House.

In order to win the presidency, a candidate must have a majority - at least 270 - of the 538 electoral votes which are awarded on the basis of state-by-state contests. In all but two states, the candidate who receives the most votes statewide wins all of that state's electoral votes.

Each state has a number of electors equal to the number of its Senators (2) and Representatives (at least 1) in the US Congress.

The nation's capital, Washington, DC, also has 3 electoral votes, even though it has no voting seats in Congress. -- AP