On the Eastern Shore, Republican Rep. Wayne T. Gilchrest faces a challenge from a pair of conservative state senators. In Prince George's County, Democratic Rep. Albert R. Wynn tries to fend off the progressive activist who nearly unseated him in 2006.
With Maryland just the second state to hold congressional primaries this year, the mirror-image races are drawing national attention.
"It's a good preview of possibly the breadth of anti-incumbent sentiment," said David Wasserman, the House editor of The Cook Political Report, a Washington-based newsletter for insiders and political junkies. "You have two races, one on either side of the aisle, that will say something about the mood of the country."
In a state where competition has been redistricted out of the general election, the votes tomorrow will likely put the finishing touches on Maryland's congressional delegation for 2009. The winners of the Republican primary in the conservative 1st Congressional District - where state Sens. Andy Harris and E.J. Pipkin are challenging Gilchrest - and the Democratic primary in the liberal 4th - where attorney and activist Donna Edwards is taking a second swing at Wynn - are expected to coast to victory in November.
The other six House members from Maryland - Democrats Elijah E. Cummings, Steny H. Hoyer, C.A. Dutch Ruppersberger, John Sarbanes and Chris Van Hollen and Republican Roscoe G. Bartlett - face no more than nominal opposition this year. Neither Sen. Benjamin L. Cardin nor Sen. Barbara A. Mikulski is up for re-election.
But Gilchrest and Wynn - veteran lawmakers now accused of having grown too moderate for the districts they represent - are facing the toughest challenges of their congressional careers.
"This race, the presidential race, is about change," said Ronald Walters, a political scientist at the University of Maryland. "That could carry over into the congressional races. It could have a strong influence in turning over the incumbents."
Harris certainly hopes so. One of the most conservative Republicans in Annapolis, the 51-year-old physician has cast his challenge as a struggle for the soul of the Republican Party.
This is "an incredibly important year for the Republican Party," he told a Baltimore County audience last week. "We got shellacked in 2006. We got shellacked for a very important reason. We forgot our Republican roots. We forgot the Republican roots" that gave Ronald Reagan his landslide re-election in 1984.
Harris, an obstetric anesthesiologist and commander in the U.S. Naval Reserve, has lined up several big endorsements for the district, which includes the Eastern Shore and parts of Anne Arundel, Howard and Baltimore counties. Supporters include former Gov. Robert L. Ehrlich Jr., two-time GOP gubernatorial nominee Ellen R. Sauerbrey and Focus on the Family chairman James Dobson.
Harris has also enjoyed the support of the conservative Club for Growth, which has sought to portray Gilchrest as a free-spending liberal.
The 61-year-old Gilchrest, meanwhile, has lined up still bigger names, beginning with President Bush, and including former House Speaker Newt Gingrich and Rep. Tom Cole of Nebraska, the chairman of the House Republican campaign operation. The maverick Republican - he has broken with Bush over the war in Iraq and clashed with the GOP leadership on environmental issues - also invokes Reagan during his campaign, saying his view of the world is similar to that of the conservative giant.
"He inspired people; he didn't revel in the culture of fear," said the nine-term congressman, a former school teacher and decorated Marine platoon leader in Vietnam.
Pipkin - a 51-year-old former Wall Street bond trader who joined the House race late - brings a populist record from 1 1/2 terms in the state Senate and a willingness to spend his own wealth to get elected. The son of an electrician and a cafeteria worker, Pipkin makes his opposition to the deregulation of electricity markets one of his signature issues both in Annapolis and in the campaign.
In Baltimore County last week, he railed against Democrats and said fiscal issues would be important in Washington in the coming years.
"We have to show people that there is a difference," he said. "That there is a party that believes in smaller government, that there is a party that believes there is not a revenue problem in government, but there is a spending problem in government."
In the 4th District, which includes large swaths of Prince George's and Montgomery counties, Edwards has been hammering Wynn for close to two years over his vote in 2002 to authorize President Bush to invade Iraq and his support for Republican-backed bankruptcy and energy bills.
Edwards, the executive director of the Washington-based Arca Foundation, who came within a few thousands votes of defeating the incumbent in 2006, says Wynn remains out of step with the district.