The most effective lines of attack to expect at Sunday's presidential debates

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Silhouettes of Republican presidential nominee Donald Trump and Democratic presidential nominee Hillary Clinton

Presidential candidates Hillary Clinton and Donald Trump face off Sunday in the second of three debates before the November election.

We've often heard that Clinton has poor judgement and Trump doesn't respect women very much (a perception not helped by Friday's revelations about his 2005 remarks), and that's likely to continue.

But the thing is, the candidates aren't really convincing anyone with their attacks. Most of the voters have already made up their mind and no matter how hard the candidates hammer away at their talking points, voters don't find the lines of attack any more effective.

That's according to a six-month running survey from marketing firm Fluent. Every week since May, they've been checking in with likely voters for the best reason to support their candidate of choice, and what lines of attack they find most effective against their opponent. The aggregate results could give a modest sense of what's important to voters, and what zingers candidates may use Sunday night in their effort to draw in the few remaining "undecideds."

The top lines of attack aren't that surprising. They're what we've heard again and again for months: For Clinton, it's her use of a private email server while she served as secretary of state, American deaths in Benghazi and that she's "not likable."

For Trump, his people see his impulsive behavior as a liability in such a powerful office; he doesn't understand government and that he's racist.

"Both campaigns have effectively established memes about (and attack vectors against) their opponents that have settled in," said Jordan Cohen, CMO at Fluent. "And both Clinton and Trump have been seemingly unable to shake them off."

What's interesting is how little those lines of attack have changed in terms of their effectiveness in six months since Fluent started the weekly survey.

In May, 16 percent of respondents said the argument that Trump doesn't understand government is the most effective against him. By October, that number rose to just 17 percent. So despite months of Clinton and her supporters hammering away at that point—and the many arguably incorrect statements Trump himself has made about the government—the same portion of voters think that's a good attack against him. No more, no less.

Around 15 percent of voters think that Clinton's role during the attack on the embassy at Benghazi is the best line of attack against her—down just one point from May.

"What we are seeing in these relatively consistent numbers every week since since the end of May is a reflection of the confirmation bias of the electorate," Cohen said. "Clinton and Trump supporters are polarized, dug-in and there is less and less chance that voters will change their minds as we get closer to election day."

Changing arguments, changing minds

Sunday is a town-hall style debate, in which the candidates will take questions from audience members as well as moderators Anderson Cooper of CNN and Martha Raddatz of ABC. The occasion offers Clinton the chance to try and show a softer side that critics say has been lacking in her demeanor.

Indeed, some 16 percent of voters have consistently said she is "not likable" as an effective attack line against her, according to Fluent's data.

The only attack that's really gained any traction in the past six months is the notion that Clinton broke the law by using a private email server while serving as secretary of the state. That's despite the fact that since then, the FBI concluded its investigation of Clinton's email server and found that "no charges" were appropriate in the case. Simultaneously, the U.S. Attorney General subsequently confirmed that the case would be closed without charges files.

We're likely to hear Trump's disrespect of women too brought up on Sunday, especially after an audio recording from 2005 surfaced, which caught the real estate mogul making lewd remarks about women. His perceived sexism was a big theme in the last debate, and only grew in intensity after he launched a 3 a.m. Twitter storm of invectives against Alicia Machado, a former Miss Universe.

Back in May, 6 percent of respondents said that Trump's disrespect of women was the most effective line of attack against him. That figure vacillated a bit, but had fallen a bit to 4 and 5 percent in recent weeks. Since the first debate—and Clinton's renewed attacks on the topic, that figure's jumped again to 8 percent, as of the Oct. 4 poll.

"This suggests that heading into Sunday's debate, doubling down on Trump's treatment of women could be an effective tactic for the Clinton camp," Cohen said.

Still some likables

The candidates' supporters are more flexible with their favorite things. Thirty-six percent of voters cite Clinton's experience as the best reason to vote for her, up a full 10 points from May. Trump has lost ground on his ability to negotiate effectively, at least in the eyes of the voters: Only 7 percent say his negotiations skills are the best reason to vote for him, down from 10 percent six months ago.

But many voters are still politically polarized: Many will vote for their candidate simply because the other is "much worse." Sixteen percent say that's the best reason to vote Clinton, 13 percent say the same for Trump.