Michigan's most contentious congressional primary is boiling over and could be decided not by war or the economy but on social issues such as abortion and same-sex marriage.

First-term U.S. Rep. John "Joe" Schwarz of Battle Creek faces a well-financed challenge from fellow Republican Timothy Walberg, a former state lawmaker and Christian preacher who has an army of supporters who back the traditional family.

Some observers think the 7th Congressional District is the ripest for an upset on Aug. 8.

"The Schwarz people are running scared," said Bill Ballenger, who edits Inside Michigan Politics, a political newsletter.

On paper, it would seem that Schwarz, a 68-year-old congressional rookie, could be relaxing this summer instead of ramping up his campaign for the final push to to the primary election.

His resume: Navy surgeon who tended to the wounded on the battlefields of Vietnam; a machine gun-toting CIA spy in Southeast Asia; an ear, nose and throat doctor; former Battle Creek mayor; and a member of the Michigan Senate for 16 years.

Schwarz also had lined up an impressive list of endorsements, including President Bush, U.S. Sen. John McCain, former Michigan Gov. John Engler and virtually all of the Michigan GOP hierarchy.

But Walberg, who served in the state House for 16 years and brags that he never once voted for a tax increase, has some potent allies as well. He's backed by both the National and Michigan Right to Life, and he is receiving big financial support from the Washington-based Club for Growth, a conservative anti-tax group that has flooded the airwaves with messages calling Schwarz "outrageously liberal."

Walberg's positions place him at odds with Schwarz, who favors a woman's right to choose an abortion and opposes a constitutional amendment to ban gay marriage -- arguing it's a matter for states to decide. Gay rights groups have endorsed Schwarz.

"Traditional values are key in this district," said Walberg, 55, of Tipton. "This is the race I wanted."

Two years ago, he and Schwarz were among six candidates in a GOP primary for the congressional seat from which Republican Rep. Nick Smith, R-Addison, retired. Walberg was among five conservatives, and Schwarz claimed the moderate position. The conservative vote split: When the votes were counted, Schwarz got 28 percent of them, while Walberg came in third, with 18 percent.

This time, it's just the two of them.

Given the district's political makeup, the winner of the Republican primary will be favored in November against the winner of a four-way Democratic primary. Those candidates are Daryl Campbell and Chuck Ream of Ann Arbor, Sharon Marie Renier of Munith, and Fred Strack of Dexter.

"In what is expected to be a small primary turnout, passion plays a huge role, and Tim's got the passionate Right to Life and conservative ground troops and conservative money, and you can't discount those," said Craig Ruff, senior fellow at Lansing's Public Sector Consultants, a nonpartisan public policy think tank.

"But Joe has the power of incumbency, and you can't discount that, either."

Walberg raised $236,340 in the most recent federal reporting quarter -- April 1 through June 30 -- and ended up with $397,043 in cash on hand.

Schwarz raised $435,354 in the last quarter and reported having $389,080 in cash.

Walberg said he has all but emptied his barn of his 8,000 yard signs and said campaign money continues to pour in.

His radio ads say: "The choice is clear: The liberals have Joe Schwarz. But if you are conservative, you have Tim Walberg."

Schwarz said the label is unfair and that he has voted to extend President Bush's 2003 tax cuts and end the estate tax.

"It's pretty amusing because if one looks at my voting record, it's pretty much pure vanilla Republican," Schwarz said.

The Michigan Chamber of Commerce agrees. Its president, Jim Barrett, is featured in radio ads saying: "Don't take seriously the claim by a few that Joe Schwarz is some kind of a flaming liberal. He scored 88 percent on the U.S. Chamber's voting record on business and tax issues."

The candidates' differences extend beyond social issues.

For instance, they come down on opposite sides of whether to drill for oil in Alaska's Arctic National Wildlife Refuge.

"The oil sitting there untapped would at least equal what we're taking in from Canada," said Walberg, adding that it's a step toward lessening the threat of Middle East oil holding the U.S. hostage.

Schwarz said it's impractical.

"It won't produce enough oil, maybe five or seven months' worth," he said. "And you wouldn't see a drop of it for at least eight to 10 years."