If Mr. Taylor is out of the race, Mr. Orman will have a chance to consolidate the anti-Roberts vote — and he is clearly better positioned to do so. Mr. Taylor was mainly winning partisan Democrats who will almost certainly oppose Mr. Roberts.

Mr. Orman, on the other hand, was showing far broader appeal, favored by 18 percent of Republicans and by 38 percent of independents in the last SurveyUSA poll. Mr. Orman, in other words, was already positioned to carry the Republican-leaning independent voters necessary to win — if only those supporters could be paired with the Democrats who now seem extremely likely to move in his direction.

The unusual circumstances create quite a bit of uncertainty, and The Upshot’s Senate forecasting model is expected to give Mr. Orman decent odds. It would not be at all surprising if the next polls show Mr. Orman with an early lead. The question is whether he can hold it.

The case for a Roberts win is straightforward. Mr. Orman is a fairly liberal candidate, and Kansans typically do not support relatively liberal candidates. He even ran as a Democratic candidate for Senate in 2008. Mr. Orman hasn’t said whether he would vote for Harry Reid or Mitch McConnell for Senate majority leader — and the possibility that Mr. Orman could cast a decisive vote for Democrats will surely be an issue.