As pundits and consultants play Monday morning quarterback to analyze what went wrong with Hillary Clinton's campaign, her online ad spending numbers may shed some light. Through the entire 2007/2008 campaign, Hillary Clinton for President spent far less on online advertising than Barack Obama's campaign has this year alone. Still, despite the disparity, they did have something in common: both spent the bulk of their online ad budgets on Google.

Google political ad man Peter Greenberger thinks Hillary Clinton's campaign was hampered by its failure to use search advertising consistently throughout the primary season. While he acknowledges the New York Senator's campaign deserves credit for experimenting with new media tools, he argues savvy search advertising contributed to the success of both Barack Obama's and John McCain's primary season wins by helping them build supporter lists. Through sponsored links in search results, the McCain and Obama camps have driven users seeking information on the candidates to their official Web sites to register for newsletters, volunteer or to donate money.

Barack Obama's campaign spent nearly $3 million on online advertising related purchases between January and April. The biggest recipient of the Democratic Presidential hopeful's online ad dollars was Google.

Obama won big in this month's North Carolina primary and his online ad strategy in April may have played a role in driving his supporters to the polls. As the primaries chug along, Senator Barack Obama has steamrolled past fellow Democratic Presidential hopeful Senator Hillary Clinton and likely Republican nominee John McCain in the Web ad department.

Hillary Clinton, Barack Obama and John McCain continue to hit up supporters for cash via e-mail, but Obama for America is going one step further. The Senator's campaign is asking diehards via e-mail to trek to Oregon and knock on doors. The grassroots-minded campaign also used e-mail to push for residents of Obama's home state of Illinois to get out Tuesday's Indiana vote, in-person.

Senator Barack Obama's campaign has demonstrated a willingness to use local online advertising to reach voters in particular primary states. Now, with the important Pennsylvania primary around the corner, Obama for America is kicking it up a notch with new issue-based ads targeted to news sites in the state. Purely focused on persuasion, the ads mark a first for the Obama campaign when it comes to Web advertising, and stand in stark contrast to fundraising ads placed by the Clinton campaign.

In February, Democratic presidential hopeful Barack Obama battled for Texas voters in millions of get out the vote ads leading up to the Lone Star primary and caucuses. Meanwhile, John McCain took advantage of his lone Republican candidate status with a "Get Hillary" Web ad campaign.

Senator Barack Obama wants voters in Texas and Ohio to vote early, and his campaign is running huge video-enabled billboard ads to promote the convenient option. Yet, despite a desperate need to beat her Democratic opponent in the two states in Tuesday's primaries, Senator Hillary Clinton's camp doesn't seem to be running Web display ads at all.

Media coverage of 2008 presidential campaigning on the Web has been dominated by talk of social networking, blogs, viral video, and other tough-to-track social media phenomena. No campaign staffer worth his salt would deny the potential impact of an Obama supporter posting a link on her MySpace page to the candidate's site. However, the fact is many of the campaigns have used a far more measurable online campaign method: paid display advertising.

Presidential campaigns may not be spending tons on Web ads, but they've recently shown a greater willingness to experiment with innovative ad creative and messaging. Though these advertisers are typically less savvy than commercial marketers, ads placed by the campaigns in December leading to the Iowa caucuses revealed a maturing approach to online advertising.