Unless the tide turns in South Dakota sometime during the next three weeks, Democrat Stephanie Herseth will win the special election for the state’s at-large House seat.

No, the race is not yet over. Democrats can’t put it in the win column, and the National Republican Congressional Committee certainly has no reason to throw in the towel for its nominee, state Sen. Larry Diedrich. [IMGCAP(1)]

Diedrich, 46, served two terms in the South Dakota House before winning a seat in the state Senate in 2000. A former president of the South Dakota Soybean Association, he is a credible contender who has won the backing of the National Rifle Association and the U.S. Chamber of Commerce.

But Herseth started out with an advantage, having run a credible if unsuccessful race against Bill Janklow (R) in 2002, and she has wisely run this race in such a way as to make it difficult for her GOP opponent to close the gap.

An attractive 33-year-old whose grandfather served as governor and whose family name is well known in state politics, Herseth has been trying to make the special election a popularity contest, and if she succeeds, she will win.

Voters selected Janklow over her two years ago, and the former governor’s legal problems and subsequent resignation from Congress surely have some South Dakota voters figuring they picked the wrong candidate in that race and that they can correct the error in judgment by now sending Herseth to Washington, D.C.

Republican Diedrich’s strategy is clear. He needs to (and is trying to) draw differences on the issues with Herseth, thereby taking advantage of the state’s conservatism and Republican bent.

If voters believe that Diedrich and Herseth hold the same views and will vote the same way on Capitol Hill, then they might as well elect Herseth. But if they believe that the Democrat is more liberal in her views, they might opt for Diedrich.

The Republican’s first TV ad tries to draw a distinction with Herseth on taxes. After noting that he and Herseth have “promised no personal attacks,” Diedrich argues that, “To make an informed decision, you also deserve a respectful debate about issues. About where we agree, and yes, where we disagree. For example, we both strongly support country-of-origin labeling. But on tax cuts, I think they should be made permanent. Stephanie does not.”

Compared to other “comparative” spots, this one isn’t merely restrained. It’s downright wimpy. The Republican not only indicates an issue of agreement with his Democratic opponent, he states the alleged difference of opinion in a straightforward, unemotional way.

In turn, Herseth responded as if Diedrich just accused her of strangling kittens. She badly overreacted, making herself the one guilty of directing a “negative attack.”

In her response spot, Herseth asserts that the Republican “tries to mislead you about my position on taxes,” a far more loaded phrase than any her opponent used.

Then, she lowers the boom on him, closing her ad with, “I approved this message because I’m committed to a truthful campaign. It’s clear that Larry Diedrich is not.”

On one hand, I can admire the Herseth ad, since she has smartly played the “negative attacks” card in her efforts to make it impossible for her opponent to draw any comparisons in the future. The state’s voters have shown their fatigue with negative attacks, and any candidate who is widely viewed as being too negative will be punished.

But on the other hand, Herseth is like the little boy who cries “wolf” when no threatening animal is in sight. And little boys (or girls) who cry wolf shouldn’t be rewarded for their supposed cleverness.

Herseth’s response is a classic effort at inoculation. She is trying to make it impossible for Diedrich to identify differences with her, even if they exist.

If Diedrich were merely misstating her position, she could simply say so and assert her support for making the Bush tax cuts permanent. But things aren’t always as clear as that with Herseth, or at least so say Republicans. They complain that her positions are fuzzy and that she adds caveats and modifiers in an effort to avoid taking a clear-cut position.

I’m certainly not going to attempt to referee this argument. It’s up to Diedrich to make his case and Herseth to fend off his charges. But I must admit that, while I was impressed with Herseth when I interviewed her, I also had an uneasy feeling about her.

Although Herseth has never held elective office, she talks like a politician (in my interviews with her, not in her TV commercials, which are very well done). She appears very cautious in selecting her words, giving the impression that she is trying to answer questions without revealing much.

But I’m not convinced that the state’s voters will punish her for her tactic, or that Diedrich will be able to draw a stark enough contrast with her to overcome her advantages. And that means the South Dakota race is Stephanie Herseth’s to lose.

Stuart Rothenberg is editor of the Rothenberg Political Report .