Maryland's Democratic candidates for the open U.S. Senate seat face a close and contentious race as voters head to the polls for the state's primary on April 26. (WUSA9)

A Senate primary that has exposed deep racial, gender and class divisions within the Maryland Democratic Party will finally be in the hands of the electorate Tuesday, as voters choose a nominee to compete in November for a rare open seat being vacated by Sen. Barbara A. Mikulski (D) after 30 years.

The race has been close and contentious for months, as two members of Congress sacrificed their safe seats to reach for the Senate and stake a claim to the progressive vote in the heavily Democratic state.

Rep. Chris Van Hollen, who has built a reputation over two decades in public office as a policy-steeped dealmaker, has been battling Rep. Donna Edwards, a black single mother with an activist history but fewer political allies.

Mikulski, the first Democratic woman elected to the Senate in her own right and the first endorsed by the now-powerful Emily’s List, is known for her sharp tongue and sharp elbows as well as her emphatic liberalism. But she’s also a canny negotiator who used her influence on the Senate Appropriations Committee to simultaneously advance her values and Maryland’s interests.

As she steps down, both candidates are claiming her mantle. Edwards says that, like Mikulski, she’s an outsider taking on a male-dominated political machine. Van Hollen says that his role in crafting major Democratic legislation makes him a power broker in Mikulski’s mode.

In recent days, Edwards has charged that criticisms lobbed by Van Hollen’s camp that paint her as the less effective lawmaker are infused with racism and sexism.

“I thought the Republican Party was full of dog whistles, but the Democratic Party has a foghorn,” she said in an interview with BuzzFeed.

That sentiment was echoed by Terry O’Neill of the National Organization of Women, which has endorsed Edwards. “[The criticism of Edwards] is quite subtle in its sexism, but it’s sexist,” she said in a conference call Friday. “When you put this out there, that strong capable women who are really trying to get something done are not playing the game the right way — that’s a double standard.”

Edwards has put race and gender at the forefront of her campaign, emphasizing that only one black woman has ever served in the U.S. Senate.

“I’m dumbfounded by Democrats who don’t see the value of race and gender as part of a mix of who we are on public and private lives,” she told BuzzFeed.

Van Hollen, meanwhile, has pushed back through surrogates, arguing that he shouldn’t be judged by his skin color or gender, but by his record and ability to deliver on the same progressive agenda.

“We are mindful of our natural affinity with Chris Van Hollen’s opponent and the paucity of racial and gender diversity in the U.S. Senate,” reads a letter from his campaign signed by 100 African American female supporters. But in the end, they say, the contest for the Senate “is not about race, gender, creed or color. It is about a person.”

While policy has often taken a back seat in the race to personality, there are a few substantive differences between the two candidates.

Edwards has voted against every trade deal to come before Congress during her tenure. Van Hollen has supported several, although he opposed multilateral deals in Central America and Asia.

Edwards has stood to Van Hollen’s left on the Israeli-Palestinian conflict, declining to sign on to letters and resolutions backed by the American Israel Public Affairs Committee. Van Hollen has supported such measures and, since catching flak for criticizing Israel’s actions in the 2006 war with Lebanon, has avoided offending the pro-Israel lobby.

But most of their differences are stylistic.

Edwards was an earlier opponent of the Trans-Pacific Partnership and an earlier supporter of Obama’s nuclear deal with Iran. While both are against unlimited campaign spending, Edwards opposed Van Hollen’s attempt to pass donor-disclosure legislation in 2010 because his plan would have exempted the National Rifle Association. Both support an expansion of Social Security and Medicare. But Edwards suggests Van Hollen was willing to compromise on some benefits during deficit negotiations. Van Hollen counters that as the ranking Democrat on the budget committee, he was the one defending the programs against Republican attacks.

Despite the candidates’ agreement on the vast majority of issues, they and outside groups have spent millions on the race.

Van Hollen has dramatically outraised and outspent Edwards. He has the support of super PACs run by the National Realtors Association and the Service Employees International Union; together they have spent about $1.5 million on his behalf.

Edwards has been bolstered by both Emily’s List and a super PAC called “Working for Us” associated with hedge-fund manager S. Donald Sussman. Each group has spent more than $2 million on the race.

The Republican Senate primary in Maryland has attracted far less money and attention. Four candidates have raised enough to make some impression on voters: Former Pentagon official Richard Douglas, tire company owner Joseph Hooe, National Association of Manufacturers executive Chrysovalantis Kefalas and state Del. Kathy Szeliga.