Republican vice presidential candidate, Alaska Gov. Sarah Palin, pumps her fist as she walks toward a bus at a campaign stop Saturday in Washington, Pa.

Beaver, Pa. ? Keep your eyes on Pennsylvania and Michigan.

There are battleground states in the presidential election, and then there are these two, looming larger than most others because they offer such a rich opportunity for Republican John McCain and potential peril for Democrat Barack Obama.

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

“I need Pennsylvania,” Obama told a crowd of several thousand at an outdoor rally in Beaver on Friday night.

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

“I’m extremely nervous,” she said of 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.

If McCain carries either state, he could lose several states that President Bush won and still claim the White House. For 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 Obama and his running mate, Delaware Sen. Joe Biden, made Pennsylvania their first stop Friday after leaving their party’s convention in Denver, with several Michigan stops scheduled for today and Monday. Their campaign released a new TV ad for northeastern Pennsylvania noting that Biden was born in Scranton.

McCain and his new running mate, Alaska Gov. Sarah Palin, campaigned Saturday in Pittsburgh and Washington, Pa., 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.

John Kerry and Al Gore carried both in their losing campaigns in 2000 and 2004. So they form an almost must-win minimum for 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.

They’re being told by Obama’s people that their personal and political interests should trump any qualms about voting for a black man in his first Senate term.

Polls show Obama slightly ahead in both states, but McCain may be within striking distance.

To be elected, Obama must win 18 more electoral votes than 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 Obama tend to be small, with few electoral votes.

The possibilities and math can get complicated.

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

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