Rep. Chris Van Hollen speaks in a candidates forum with Rep. Donna Edwards on March 18 in Greenbelt, Md. (Mark Gail/For The Washington Post)

A super PAC run by the National Association of Realtors has spent over half a million dollars in support of Rep. Chris Van Hollen’s Senate campaign.

The money has gone toward television and Internet advertising as well as polling in the tight Democratic primary between Van Hollen and Rep. Donna F. Edwards. The group would not disclose how much money it intends to spend or where. However, so far the group has bought time in the Baltimore television market.

“Maryland needs and deserves proven, effective leadership,” its 30-second ad says. Van Hollen, the ad says, “has delivered that leadership” by encouraging affordable home ownership.

NAR spokesman Jon Boughtin said that the group “makes targeted investments in campaigns at the request of local and state associations who help identify candidates, like Congressman Chris Van Hollen, who have a strong track record of support for homeownership.”

He would not say how much the group plans to spend on the race beyond the $554,850 already reported.

The super PAC is the first outside group to support Van Hollen in the Senate race. For months, Edwards has benefited from the backing of a super PAC run by the Democratic abortion rights women’s group Emily’s List. Early in the race, Van Hollen had pushed Edwards to sign a pledge barring such outside groups from the race, but she refused.

Van Hollen’s pledge would have required both candidates to donate to charity an amount equivalent to half what any outside group spent on the race. But he would not commit to such a pledge unilaterally.

“Chris Van Hollen is the only one in this race who has taken a stand against outside Super PAC money flooding into Maryland,” Van Hollen spokeswoman Bridgett Frey said in a statement. “He proposed an Elizabeth Warren-style anti-Super PAC pledge to keep them out of this election — the offer was quickly rejected because Congresswoman Edwards has a habit of saying one thing and doing another.”

Edwards spokesman Benjamin Gerdes countered that the investment was a sign of “Washington special interests” taking Van Hollen’s side in the race.

Van Hollen has outraised Edwards in the race by a wide margin. At the end of last year, he had more than 10 times as much money in his campaign account. He has advertised extensively in Baltimore; she has not aired any ads at all. But Women Vote, the Emily’s List super PAC, has committed at least $2.4 million to supporting Edwards with advertising on television and online.

The most recent polling in the Senate race gives Edwards a slight lead, while several other recent polls find the two candidates tied. Their first debate was last Friday; a second debate is scheduled for this Friday.