“In an inexpensive market like Topeka or Joplin, an independent group with minimal resources can come in, own that market for three weeks and can have a major impact on a campaign,” said Chris LaCivita, a strategist for Senator Pat Roberts, Republican of Kansas, who is seeking re-election and has been bolstered by millions of dollars in spending by groups formed in recent weeks.

At least 90 political nonprofits, super PACs and outside groups did not spend any money until the beginning of October, according to a New York Times analysis. Among them are 18 groups that did not exist in September, and that had collectively spent almost $9 million through Thursday.

Several are spending money they did not have on Oct. 15, the last day before Election Day for which super PACs must disclose contributions. One late-spending super PAC called Arkansas Horizon has spent more than $1 million in the last two weeks on ads attacking Senator Mark Pryor, a Democrat — far more than the cash on hand it reported as of mid-October.

In Kansas, a group called the Alliance for a Free Society has dumped almost half a million dollars into the Senate race there, attacking the independent candidate, Greg Orman. In one ad, a man identified only as a Kansas rancher named John attacks Mr. Orman as a closet Democrat who is “not being honest with us.”

Image The Alliance for a Free Society bought a Kansas ad, seen on Vimeo.

The alliance, registered in Delaware as a nonprofit corporation, was formed in July, and until recently did not even have a website. Documents filed with the Federal Election Commission indicated that the group was connected to a former executive of Koch Industries, the Kansas-based conglomerate owned by the brothers Charles G. and David H. Koch.

Yet another new super PAC, American Future Fund Political Action, reported having just $56,000 in mid-October, according to election commission reports. But on Tuesday, the group announced that it would spend more than $300,000 on behalf of Representative Fred Upton, Republican of Michigan, chairman of the House Committee on Energy and Commerce.