Barack Obama is shown on TV during his 2008 campaign for president, but watching live TV is no longer as common as it was then, and campaign strategists are having to adjust. (NICHOLAS KAMM/AFP/Getty Images)

For half a century, television ads have been the staple of political campaigns, the preferred, if costly, vehicle for communicating a candidate’s message to the voters. What happens when people stop watching live television?

That day hasn’t arrived yet and probably never will. But the outlines of the new world of television watching habits — and their implications for political campaigns — were highlighted in a survey released Thursday at a conference hosted by Harvard’s Institute of Politics and the Internet Association.

The survey, presented by Robert Blizzard of the Republican firm Public Opinion Strategies and Julie Hootkin of the Democratic firm Global Strategy Group, concluded that the country has reached “a tipping point” in the competition for viewers between traditional live television and other forms of viewing content.

“That means, for political campaigns, reaching younger, more diverse, swing voters through live TV advertising alone is problematic,” the authors wrote in their analysis.

This is the third such survey in the past four years. For the first time, fewer than half (48 percent) of all voters say that live TV is their primary source for watching video content. The second-most-preferred form for viewing is through recorded programming, but a majority said they skip 100 percent of the ads when they watch.

Live TV isn’t going away; it’s just not as dominant as it once was. Seventy percent of those surveyed said they had watched live television in the previous week. But fully 30 percent said that, other than live sporting events, they had watched no live television in the previous week. For younger voters, it’s closer to 40 percent.

Video on demand, streaming, smartphones and tablets have changed viewing habits. In the past three years, according to the survey, the percentage of people watching streaming content — think “House of Cards” on Netflix — has roughly doubled, to 27 percent of the population. Viewing content on smartphones has about doubled to roughly the same percentage of users. Tablet viewing has jumped from 14 percent to 26 percent in less than two years.

These changes in viewing habits coincide with the dramatic growth in the prevalence of smartphones and tablets. Today, two-thirds of the population has a smartphone; more than half said they have a tablet.

Smartphones were not in general use during the 2008 campaign, when President Obama’s team supposedly broke the mold in its employment of new technology. By the 2012 campaign, the smartphone became a hot new platform not only for the delivery of political messages and content, but also for fundraising.

Television ads are still the best way to reach large numbers of people, but as the audience continues to fragment and viewing habits change, campaigns are being forced to diversify the ways in which they deliver their messages.

The recent special congressional election in Florida’s 13th District, where Republican David Jolly defeated Democrat Alex Sink, is a classic example of one problem that confronts campaign strategists in House races. The Tampa media market sprawls over eight congressional districts, but the 13th District accounts for just 19 percent of the total population in that market.

A post-election analysis posted by the Smart Media Group said combined Democratic spending on television totaled about $5.5 million, while combined Republican spending totaled $4.4 million. About three-quarters of that was spent on broadcast networks, and most of the rest was spent on cable.

That means millions of dollars spent on local broadcast channels were essentially wasted because most of the people watching did not reside in the 13th District. Whether the money spent on cable was any more efficiently allocated isn’t clear.

That is far less a problem for presidential campaigns because they’re generally focused only on media markets, not congressional boundaries. But they too face big questions about how efficiently they are spending their ad dollars and how effectively television advertising is in changing minds.

“The objective is reaching voters where they are,” Jim Margolis, Obama’s media adviser, wrote in an e-mail. “Content is content and whether you see an ad or video on your iPad, your TV or on your smartphone, our job is to get in front of your eyeballs and get your attention. That means looking for gaps in TV penetration, and finding targets someplace else.”

Even before the arrival of smartphones, tablets and streaming content, the proliferation of cable channels diminished the power and reach of the broadcast networks as a vehicle for political advertising. Audiences are more fragmented than ever.

Finding those viewers is becoming increasingly difficult, though the new tools available to campaigns make tailoring messages easier. Margolis cited analytics, information gathered from set-top boxes, online tracking and searching as ways “to deliver specific ads to specific boxes.”

Obama’s campaign was far more sophisticated than Mitt Romney’s in using these new tools to analyze viewing patterns and the demographics of viewers on particular channels and programs. They ran their ads across a broader array of cable channels than ever with an eye to reaching specific groups of voters, particularly swing voters.

The coming challenge, outlined in the new survey, suggests an acceleration in how people prefer to get their video content. Over time, this will require a substantial reorientation in thinking about how to allocate advertising dollars.

Some figures supplied by Margolis’s firm, GMMB, show how rapidly the Obama campaign adapted its spending in just four years. In 2008, the campaign spent $253 million on television ads and $16 million on its digital efforts, including fundraising. In 2012, the campaign spent $380 million on television and $112 million on digital.

The Obama campaign was more digitally focused than any in modern times. But in most campaigns, digital spending is still relatively small. Commercial firms have been quicker to adapt than campaigns, according to experts. Many commercial firms now spend 20 to 30 percent of their ad dollars on digital. Most political campaigns spend around 5 percent. This will soon change.

New technologies, massive amounts of data, and the employment of analytics and modeling are changing politics and creating strains inside campaigns. A new generation of strategists, who came of age in the digital era and are believers in the new techniques, is demanding a bigger share of the campaign budget and more authority in decision-making.

Margolis wrote that campaigns will continue to spend more and more on their digital and mobile efforts, but he added that, for the time being, TV will continue to dominate. But there are some people now working in campaigns who question whether television ads, particularly in presidential campaigns, are as capable of changing voters’ minds, as has long been believed.

That’s a topic for another time. For now, everyone in politics recognizes the transition that is underway in delivering messages, positive and negative. As Margolis put it: “To do it right requires more work and sophistication than when people were watching four local stations and we were bombing them . . . with ads. The advertising world has changed in powerful ways and reaching voters is more challenging than ever before.”

If this is truly a tipping point — and everything suggests that the changes in viewing habits will accelerate — then campaign strategists will have to change their habits even more rapidly than they may think.