His chances would be better if Mr. Weiland were to lose significant support, but that does not look likely. Mayday.us, a crowdsourced super PAC that has raised nearly $8 million to support candidates who support campaign finance overhaul, is already airing pro-Weiland advertisements. On Wednesday, Bloomberg News reported that the Democratic Senatorial Campaign Committee would spend a million dollars on campaign advertisements on behalf of Mr. Weiland. If true, the investment would be significant in one of the country’s least populous states.

The big question is the extent to which these advertisements will support Mr. Weiland or attack Mr. Rounds. If the advertisements promote Mr. Weiland, that would be bad news for Mr. Pressler. But if the advertisements attack Mr. Rounds, Mr. Pressler would be just as likely to benefit as Mr. Weiland — maybe likelier, since Mr. Rounds holds fairly reliably Republican voters who would be unlikely to support a Democrat under any circumstances.

Mr. Weiland’s path to victory will be challenging, even if Democrats make a substantial investment on his behalf. He hasn’t eclipsed 29 percent of the vote in a survey that wasn’t sponsored by his own campaign, while Mr. Rounds has never fallen beneath 35 percent. Mr. Weiland probably can’t peel support away from Mr. Rounds, so he would need to force Mr. Pressler beneath 25 percent, or lower. Mr. Pressler’s recent strength makes that less likely.

But not impossible. Mr. Pressler’s resources are so limited that it’s possible to imagine him fading over the last few weeks. President Obama won 40 percent of the vote in South Dakota in 2012, so it’s not at all impossible to imagine a Democrat getting up to the number needed to prevail in a three-way race.

But the biggest reason to be cautious is that three-way races are particularly unpredictable. Fairly significant polling errors occurred in the three 2010 three-way statewide contests (defined as a contest in which three candidates entered Election Day with at least 20 percent of the vote in most polls). In governors' races in Rhode Island and Maine, the error averaged about 8 points; in Alaska, the majority of pre-election polls showed Joe Miller in the lead, but Lisa Murkowski prevailed by about four points.

With this history and the race beginning to attract national spending, it wouldn’t be wise to dismiss anyone’s chances.