IOWA CITY — Over pizza and cupcakes, Jose Nunez implored a crowd of University of Iowa students to pull out their smartphones and start sharing with online friends their personal reasons for backing Hillary Clinton for president.

Early voting in Iowa begins in late September, and the digital aide for the Clinton campaign said earnestly: ‘‘These stories are going to start making a difference.’’

It’s a scene Clinton’s campaign is replicating in about a dozen of the most contested states as their aggressive voter turnout effort kicks into high gear for the sprint to Election Day, Nov. 8.

In Iowa alone, 25,000 volunteers are helping send real-time data on voters back to the campaign’s New York City headquarters, where dozens of analysts seek to predict the electorate’s direction.

The campaign says it has about a half-million volunteers in swing states.

Clinton’s team believes winning the 270 electoral votes needed to secure the presidency relies on its yearlong, multimillion-dollar investment in sophisticated analytics and an army of volunteers now making contact with individual voters.

Trump, with heavy assistance from the Republican National Committee, is trying to replicate some of those efforts, bringing staff on and hoping they can make up for lost time.

On Saturday, the RNC expected to have about 1,000 employees and 4,000 volunteers knocking on doors in Ohio, Florida, Wisconsin, and elsewhere.

But the billionaire businessman — who said earlier this year that he believes the use of data analysis in campaigning is ‘‘overrated’’ — is also banking on a wave of momentum to carry voters to the polls.

‘‘This is an incredible movement. The world is talking about it,’’ Trump said during one of his rallies, which have served as a main way of collecting information about voters.

RNC officials talk publicly about having made gains since President Obama won reelection in 2012, and there’s no doubt the party has focused more on voter targeting in recent years.

‘‘We know we have to be better than we have been before to win a tight election,’’ said Rick Gorka, a communications adviser to the RNC and a former campaign aide to Mitt Romney, the 2012 nominee.