Martin O'Malley's Passive-Aggressive Campaign Against Hillary Clinton

The underdog won't say Clinton's name, but behind the scenes, he's going after her.

Martin O'Malley has lots to say about his support for same-sex marriage. He'll happily offer criticism of the Pacific trade deal dividing Democrats, and he's always poised to talk about the source of real leadership being principles not polls.

But there are two words he's trying hard to avoid saying in public: Hillary Clinton.

O'Malley, who is expected to announce next month whether or not he will challenge Clinton for the Democratic nomination, is doing everything he can to avoid uttering her name. That doesn't mean he's not drawing contrasts, though.

(RELATED: Labor Makes Trade a Line in the Sand for 2016. But What If Hillary Clinton Crosses It?)

O'Malley and his aides are keen to talk about his progressive positions on policy questions dogging Clinton on any given day. Indeed, his team is even quick to suggest—via unsolicited emails to Clinton beat reporters—specific issues for the media to take up with the front-runner.

But all that's behind the scenes. On the record, the former Maryland governor has been deliberately keeping his criticism and comparisons with Clinton implicit, in the hope of impressing voters with a substantive platform rather than making himself memorable by bashing his opponent.

Just this weekend, he told South Carolina activists that "leadership is about forming a public opinion, not about chasing after it." He went on: "It's not about the polls. It's about our principles."

(RELATED: Hillary Clinton May Have Lost a Campaign Weapon)

But, it's also about polls. Especially for O'Malley, who needs all the attention he can get. He's little-known across the country and barely registers in national and early-state polling. A Quinnipiac national poll from earlier this month put O'Malley at just 3 percent, behind Vermont Sen. Bernie Sanders and far behind Clinton at 60 percent. Another poll, from CNN/ORC, had O'Malley at a dismal 1 percent.

His team, though, thinks he gains more lasting attention by drawing indirect contrasts.

Last week in New Hampshire, for example, Clinton was asked about her position on the Trans-Pacific Partnership, a trade agreement that is now dividing Democrats but that she supported when part of the Obama administration. She replied that "any trade deal has to produce jobs and raise wages and increase prosperity and protect our security."

(RELATED: Here Are All Seven Media Questions Hillary Clinton Has Answered During Her Campaign)

O'Malley's PAC responded by sending an email to supporters the next day titled "Hard Choice?"—a clear reference to Clinton's memoir Hard Choices—in which he opposed the deal. "American workers whose jobs could be on the line right now are owed more than lip service," O'Malley wrote in that email.

He took a similar tack on two other issues that open Clinton to charges of flip-flopping—same-sex marriage and drivers' licenses for undocumented immigrants—by releasing videos or sending out emails to his PAC's list in response to her statements from the trail.

This strategy lets O'Malley project himself to voters as policy-focused rather than Clinton-focused. "It's going to be very difficult for O'Malley to draw real contrasts with Hillary Clinton without alienating the vast majority of the Democratic base," said Bill Burton, a California-based strategist who served as President Obama's spokesman in 2008. "There may be an appetite for a more progressive argument inside the Democratic Party, but for the majority, they won't think it's worth it if it is detrimental to the Clinton campaign."

(RELATED: The Proof of Clinton's Wrongdoing)

What's more, voters don't even hear O'Malley's Clinton contrasts in the same way as Beltway insiders do, contends veteran Democratic strategist Steve McMahon. "It may be obvious to the media, but it doesn't sound negative to the voter," he said.

This calculation also is keeping O'Malley quiet on the controversies surrounding foreign donations to the Clinton Foundation, Clinton's unauthorized private-email server, and her decision to delete 30,000 emails from her time at State that she did not want the public to see.

"The Republicans are coming out of the woodwork to go after her," said George Appleby, a Des Moines-based lawyer who backs O'Malley. "He doesn't have to do anything—he doesn't have to go after Clinton in that way."

(RELATED: Mark Your Calendar, Hillary Clinton. You Have Two Pending Dates With Trey Gowdy)

There's one more reason for O'Malley's continued focus on the issues that separate him from Clinton: It could help him pick up Elizabeth Warren's supporters. The progressives urging the Massachusetts senator to run have built an infrastructure in Iowa and New Hampshire on her behalf but will likely be looking for an alternative now that it's becoming clear she won't enter the race.

So far, Warren's supporters have been reluctant to back another candidate, but O'Malley's team thinks he could be their natural next choice.

"The fact of the matter is, there is a group of progressives out there that are open, open to something forward-looking," said Bill Hyers, the O'Malley senior adviser who engineered Bill de Blasio's victory in the New York mayoral race in 2013. "They're not just going to naturally fall into a camp just to fall into a camp. "... [But] he's got a message that will probably appeal to people who are open to a different kind of message."

Certainly, there would be benefits to attacking Clinton outright, namely attention, lots of headlines, and a probable bump in the polls. Former Rhode Island Gov. Lincoln Chafee, for example, who is also considering a campaign for the Democratic nomination, has garnered media attention for coming out strongly against Clinton on foreign policy issues.

But Democratic strategists say that only works for so long—and there's a real chance that running hard against Clinton could backfire, which is why O'Malley is largely sticking to veiled shots.

"You don't want to be the one candidate out there with a meat axe," said longtime Democratic strategist Steve McMahon. "The aggressor "... in a multicandidate field is usually not the one who benefits from the aggression—it's somebody else. The O'Malley campaign seems to understand that, and they're building their strategy around it."

Whether O'Malley can parlay his issue-focused message into tangible support remains to be seen. His team says he'll be spending lots of time on the ground in the early states, hoping that retail politics will help him slowly chip away at Clinton's seemingly insurmountable lead. Early-state activists suggest there's room for a Clinton alternative, but that it will be a hugely uphill battle.

Still, Boyd Brown, a Democratic National Committeeman from South Carolina, put it this way: "He's got nowhere to go but up."