Maryland Lt. Gov. Anthony G. Brown is countering Attorney General Douglas F. Gansler's proposed pledge to forgo third-party advertising in the 2014 Democratic gubernatorial race with a suggested promise of his own: Let's be nice.

Brown, who has been under pressure from Gansler for more than a week to agree to discourage groups that support the lieutenant governor from running ads on his behalf, challenged the attorney general and Del. Heather R. Mizeur of Montgomery County Thursday night to sign on to a "positive campaign pledge" under which each would agree not to attack the other two.

Brown contended that Gansler's proposed pledge "missed the mark."

"What Marylanders find most disheartening about political campaigns is the endless stream of negative attack ads, regardless of whether they come from independent third parties or the candidates themselves," he said in a statement released by the campaign.

Brown's campaign contends that the enforcement mechanism in Gansler's proposed bargain violates Maryland law because it would require campaigns to make charity donations out of campaign funds if a third party ran ads on their behalf. The Brown campaign pointed to rulings by the Attorney General's Office that charity donations are not a proper use of campaign funds.

The State Board of Elections has agreed.

Brown, saying he appreciated the intent of Gansler's proposal, contended his plan would be legal and clear and would let candidates focus on the issues.

Reactions to Brown's late challenge are expected Friday.