Voters who click on President Barack Obama's campaign website are likely to start seeing display ads promoting his reelection bid on their Facebook pages and other sites they visit. Voters searching Google for information about Mitt Romney may notice a 15-second ad promoting the Republican presidential hopeful the next time they watch a video online.

The 2012 election could be decided by the campaign that better exploits voters' Internet data.

Strategists say the most important campaign breakthrough this year is "microtargeting," which has been a staple of product marketing. Now it's facing the greatest test yet of its political impact.

"It's easy to collect data and see what's resonating and not resonating with voters," said Adam Berke, president of the digital retargeting company AdRoll.

Campaigns have worked for years to target subsets of voters using commercially available demographic data, shopping preferences and television viewing habits. But the growing sophistication of data mining tools has given politicians an unparalleled ability to personalize messages for individual voters.

Officials in both presidential campaigns declined to discuss their digital strategy, but a review of their most recent Federal Election Commission reports shows the Romney team spent nearly $1 million on digital consulting in April. Obama spent at least $300,000.

The Obama campaign on Wednesday unveiled Dashboard, a new tool for field organizers and volunteers to collect data about voters both online and in person and deliver it to a centralized campaign database.

Campaigns use microtargeting to identify potential supporters or donors using data gleaned from a range of sources, especially their Internet browsing history. A digital profile of each person is created, allowing the campaigns to find them online and solicit them for money and support.

By layering additional data about the person who clicks on a campaign ad, such as gender or geographic location, the campaign can tailor a specific message to get that person's attention.

"Campaigns used to look at search advertising only if they could raise money off it or use it as a substitute for direct mail. We're now seeing campaigns use search ads for persuasion and mobilization," said Rob Saliterman, a former aide to President George W. Bush who handles Google's advertising sales and outreach efforts to Republican campaigns.

Campaigns will place display ads on websites targeting a voter's interests unrelated to politics, such as nature or sports or cooking.

The video-sharing site YouTube has become a popular site for campaign advertising as more people migrate from watching live television to viewing shows and other videos online. Google, which owns YouTube, receives its largest share of political advertising revenue from YouTube ads, Saliterman said.