The most effective at tacks in politics are those that stop your opponent from campaigning in his or her usual style.

When Democrats called Richard Nixon “negative” in the runup to the 1960 presidential, it made it much more difficult for him to wage the type of slash-and-burn campaign that had animated his past races. When Republicans called Bill Clinton a “flip-flopper” during his first term, it made it harder for him to reach out to all constituencies and reach across ideological barriers as he instinctually always wanted to do.

Now, Ken Mehlman, the chairman of the Republican National Committee, has pinned the “angry” moniker on Hillary Clinton – a label that will increasingly stop her from venting her partisanship as she must to get nominated.

The genius of the Mehlman charge (doubtless drawn from focus group or survey research) is that it rings so true among those who follow Sen. Clinton closely that it seems self-evident.

When Hillary denounces the deficit or wiretapping or drug prices or the administration’s inaction on global climate change, she sounds, looks, and acts angry. And the reason is that she is angry.

Hillary takes her political positions very seriously and personally. She has a hard time seeing virtue in those who disagree with her. What others would dismiss as honest disagreements about how to accomplish good ends, she often looks at as a clash between good and evil, selflessness and selfishness, generosity and greed. (She once asked how someone could “be a Republican and a Christian at the same time.”)

In her speeches and interviews, she has two speeds: bland and shrill.

When she has no sharp ideological or substantive point to make, she relaxes and acts casual – tossing her head, giggling, feigning intimacy with the interviewer.

But when she has something to say, the passion burns inside her and metastasizes into anger and thence to shrillness. Like Bella Abzug before her, Hillary can’t speak about issues without coming across as harsh and angry. Mehlman captured that affect perfectly in his characterization of Hillary as “angry.”

Bill Clinton, on the other hand, is gifted with a wide range of political expression. He can convey passion and commitment without raising his voice or gesticulating wildly with his arms. The raised eyebrow, the lilt in his voice, the sarcastic reference all do the trick. He does not need to come across as angry to make a point.

He is by far the more articulate of the couple and has merged his genial persona with political rhetoric in a way Hillary has never learned how to do.

For Hillary, there is only the sound-bite, hyperbolic, aggressive, podium thumping, rhythmic partisan rhetoric – the kind typical of Ted Kennedy. That or bland nothingness.

Hillary’s problem is that she has to run for president and make political points. But if she does so at the expense of her own popularity, she’s in a no-win situation.

The fact is that Hillary has always gained in popularity by keeping quiet. Her “up” periods, when she gained in popular approval, were all accompanied by the sounds of silence. Her global tours after the health-care plan failed; her listening tour of New York state; her opening years in the Senate – all were characterized by a silence broken only by bland, vanilla interviews in which she worked hard at saying nothing.

But when Hillary has to speak out, she usually drops in the polls. During the early days of her husband’s first run for the White House; when she tried to sell health-care reform in 1993-’94, and over these past few months, as she tries to lead her party, her outspokenness has worn poorly with the public.

Six months ago, the Fox News poll had Hillary’s favorability up to 54 percent. Now it’s down to 49 percent (with 45 percent rating her unfavorably).

The circumstances of Hillary’s presidential candidacy seem to make her nomination inevitable. But her lack of political or platform skills may prove to be a serious and perhaps lethal handicap.

She’s absolutely got to develop a third style – something between a smile and a bark.

Can her handlers teach an old dog new tricks?