Maryland Lt. Gov. Anthony Brown, left, and Attorney General Douglas Gansler. (Patrick Semansky/AP / Ricky Carioti/The Washington Post)

First came a proposal from Attorney General Douglas F. Gansler, asking fellow Democrats in Maryland’s race for governor to sign a pledge designed to discourage outside spending on the contest.

Then on Friday, Lt. Gov. Anthony G. Brown countered with a plan of his own, one that would require him and his rivals to forgo negative ads.

Both were ostensibly well-meaning attempts to inject a measure of civility into the campaign. But instead, the pledges have had the opposite effect: They increased the hostility between the two leading candidates in what is becoming an increasingly bitter race.

It didn’t take long for both proposals to be rejected by the opposing sides, leading to a rapid-fire round of finger-pointing of the kind Brown and Gansler said they were trying to avoid.

“A political ploy” by a candidate unwilling to stand up to special interests is how Gansler spokesman Bob Wheelock dismissed Brown’s proposal.

Brown’s campaign manger, Justin Schall, shot back, saying Gansler’s immediate refusal to forgo negative ads “says a lot about the kind of campaign that Gansler plans on running.”

The salvos underscored the animosity between the candidates in a Democratic primary that many believe has the potential to be every bit as ugly as the just-concluded general election contest in Virginia between Terry McAuliffe (D) and Ken Cuccinelli II (R).

This week, Gansler launched the first attack ad of the Maryland race. The Web video mocks Brown for his leadership of the rocky rollout of the state’s health-care exchange, which was set up in response to the federal health-care law. The ad is part of an effort by the Gansler camp to portray Brown as a lieutenant governor who has accomplished little and botched what he has attempted.

Brown’s camp responded by saying that Gansler is “desperate” and that “he sounds like Republicans attacking Obamacare.”

A third Democrat in the race, Del. Heather R. Mizeur (Montgomery), has largely stayed out of the fray, and her campaign manager said Friday that voters are witnessing “an arms race of political infighting and insider jabs” between Gansler and Brown.

Donald F. Norris, the chairman of the University of Maryland Baltimore County’s political science department, said he was skeptical that either Gansler or Brown is trying to accomplish his stated aims with the proposed pledges.

“I don’t think either one was about anything but political maneuvering,” Norris said. “It’s about who’s trying to get advantage over whom and how. . . . At the end of the day, both campaigns will have their pledge to wave around and say, ‘I asked them to behave themselves, and they wouldn’t agree.’ ”

Gansler offered his pledge last week after he was the subject of unflattering news stories detailing Maryland State Police allegations that he ordered troopers to drive recklessly — bypassing traffic with lights and sirens in non-emergencies — and questioning his response to a teenage beach house party he attended where there appeared to be underage drinking.

Gansler’s pledge is intended to discourage independent entities from paying for television, radio, online or direct-mail advertising that names any of the candidates. If such spending occurred, the candidate who benefited would have to pay 50 percent of the cost to a charity chosen by the other contenders.

Brown probably would have the most to lose if Gansler’s pledge were adopted. He has been endorsed by several labor groups with a history of spending money to help preferred candidates. The state’s largest teachers lobby, which endorsed Brown last month, has already made some expenditures on his behalf, buying Web ads.

Gansler’s proposal is very similar to an unprecedented agreement that was adopted during last year’s U.S. Senate race in Massachusetts between Scott Brown (R) and Elizabeth Warren (D).

In some respects, the much-hyped “People’s Pledge” was seen as a success. Super PACs and other third-party groups largely stood down.

But the tone of the race, which Warren won, was hardly civil. In the closing months, CNN reported that it had “morphed into one of the ugliest” contests in the nation. The two candidates spent more than $77 million on the race, making it the most expensive Senate contest in the country, according to the Center for Responsive Politics. (A fraction of that is expected to be spent on the June 24 Democratic primary in Maryland.)

During the past week, there were questions about whether Gansler’s proposed pledge would be legal in Maryland. Under state election law, active campaigns are barred from making charitable contributions in most circumstances.

After the Gansler campaign was reminded of that by a state election official, Wheelock argued that such a pledge could still work because state law allows campaign committees to donate to charities once they have shut down and paid off all other debts.

On Friday, Wheelock said the Gansler campaign would be happy to negotiate another arrangement, one in which money doesn’t go to charity if Gansler’s pledge is otherwise adopted.

“We are happy to work out the details,” Wheelock said. “Marylanders deserve to know where you stand.”

But Brown’s campaign had already dismissed it, unveiling a plan of its own Friday morning.

“While I appreciate the intent of Attorney General Gansler’s proposal, frankly I think what Marylanders are most sick of is negative attacks ads, so we’re raising the bar with our ‘Positive Campaign Pledge,’ ” Brown said.

In a statement rejecting Brown’s proposal a few hours later, Wheelock said the Brown campaign employs a “tracker” who films Gansler at public events “hoping to catch some ‘gotcha’ moments his campaign can then peddle to the media.”

“So, no, we aren’t signing something whose ‘positive’ spirit has already been violated by the Brown campaign months ago,” Wheelock said.

Meanwhile, Mizeur seemingly hopes to benefit from the squabbling between Brown and Gansler. Joanna Belanger, Mizeur’s campaign manager, said, “Heather is already running a positive campaign on the issues and her record.”

Unlike Mizeur’s two better-known and better-funded rivals, “we are focused on ensuring Annapolis works for all Maryland families,” Belanger said.