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The “super PAC” supporting Hillary Clinton is preparing to reserve roughly $70 million in television ads to begin after the Democratic National Convention and to run through the fall general election in important battleground states such as Ohio and Florida, the first large-scale mobilization that any group has made.

Over the coming days, the group, Priorities USA, will begin making reservations in those states, as well as in Colorado, Iowa, Nevada, New Hampshire and Virginia, said the group’s chief strategist and co-chairman, Guy Cecil. The aggressive move comes as the super PAC looks to the fall and what could be a crowded market of television ad reservations.

But it emphasizes a level of preparedness at a time when Republicans remain deeply fractured over their prohibitive front-runner, Donald J. Trump. And while they are sticking with the traditional general election battlegrounds for now, they are leaving open the possibility of expanding to other states.

Saying that Republicans have been preparing for a billion-dollar campaign to “distort Hillary Clinton’s record,” Mr. Cecil added, “Hillary’s campaign is not taking anything for granted, including the primary, which is why it’s even more important that we prepare for the long road ahead to November.”

He added: “We have learned a valuable lesson from Republicans during the primary. We cannot take Donald Trump or any other Republican nominee for granted. It will take an unprecedented campaign and independent effort to communicate, register, mobilize, and turn out voters. There’s no time to waste.”

The group is preparing for either Mr. Trump or Senator Ted Cruz of Texas. And Priorities USA is also likely to be on the air before the Democratic convention in late July. The group’s strategists are trying to determine what can be gleaned from what was effective, and what wasn’t, in the Republicans’ efforts to derail Mr. Trump.

But the $70 million is strictly for the postconvention ad buy, and that does not cover a radio and digital advertising component. By buying time early, Priorities USA hopes to target its advertising to precise shows, and, to potentially lock in better ad rates.