Online ad inventory is tight in Iowa. Early buys from Mitt Romney’s camp, some made months ago, are undoubtedly contributing factors, but even President Barack Obama’s reelection campaign is horning in. The Obama for America team bought up the DesMoinesRegister.com homepage today.

Meanwhile, the web savvy Romney campaign locked up local Iowa inventory on YouTube and has a strong presence on Des Moines Register, as well as UnionLeader.com, a key news site covering the first-in-the-nation primary state of New Hampshire.

Still, the Obama campaign managed to snap up the coveted Des Moines Register homepage today. “The Republican candidates are leaving Iowa but their terrible plans are here to stay,” declares a large takeover ad that dominates the perimeter of the Iowa news site’s homepage. The ad – which appears to be served nationally rather than only to Iowans – links to a page encouraging voters to “Join the 2012 Campaign in Iowa.” In 2008, Obama won the Democratic caucuses in Iowa, an important swing state he hopes to win again this year. The Obama for America team aims to build on an already strong base of support in the state, left over from the President’s 2008 campaign.

The page describes “the Republican plan” as one including “Hundreds of thousands in tax cuts for millionaires at the expense of the middle class, letting Wall Street write its own rules again, sending our troops back to Iraq, and more.”

Last week, one digital media buyer working for a Republican campaign rivaling Romney called Des Moines Register inventory “extremely tight right now,” adding that the site’s Caucus section was sold out. However, he said, CPM prices were “still good,” meaning not too high.

Ads for Mitt Romney can be spotted in the Iowa Caucuses section of the paper site. “America Needs a Conservative Business Leader,” state the ads, suggesting that people caucus for the candidate. Ron Paul is also there with video-enabled display ads touting his pro-life stance. And, as he has been for weeks, Paul has been running display ads on TheIowaRepublican.com, most recently promoting a pro-life message.

Michele Bachmann’s campaign is also targeting ads to Iowa voters on Des Moines Register, according to a source inside the campaign. Bachmann and Romney are pointing mobile search ads to Iowans searching for “Iowa Caucus,” according to Rob Saliterman, Google’s political ad sales exec.

The Paul and Romney campaigns – which expect to do well in the caucuses today – are already looking ahead to New Hampshire. Ads from both can be seen today on the Union Leader site. Romney ads suggesting “It’s time to turn America around” dominate the site’s homepage as Paul continues to hammer away with anti-Newt Gingrich message his campaign has been running online for weeks.

Digital media spending in Iowa by so-called Super PACs, however, has been weak. Besides a small $450 display ad buy on TheIowaRepublican.com from theRed White and Blue Fund on behalf of Rick Santorum, just one outside group seems to be making a splash online. Endorse Liberty, a newly-formed pro-Ron Paul group, appears to be throwing its entire ad budget online. According to FEC reports, the organization spent more than $207,000 on Google search and Facebook ads last week, though it’s unclear whether all the ads are hitting Iowa alone.

Iowa voters may have spotted expandable display ads featuring video of a TV spot touting Romney’s conservative businessman credentials, one which the campaign is running on television in the state in tandem with the web buy. Rarely does the campaign run a TV ad without backing up that same message online via display and video advertising, and email. Romney and most other political advertisers use pre-roll video to mimic and amplify their television messages. The goal is to reinforce the message the former Massachusetts governor is putting forth on the campaign trail, in TV ads, and elsewhere.

Targeting mobile advertising to the same degree possible online is difficult. Romney is running Zip code-targeted search, display, and video ads through Google’s AdMob. Some of those link to videos instructing voters how to caucus for the candidate. Ideally, those ads aren’t wasted on people who plan to caucus for another candidate.

The Romney team bought up YouTube pre-roll ad inventory targeting Iowa and New Hampshire residents two months ago. It’s a meticulously plotted digital media strategy observers can expect to be carried through in each important primary state. While Romney owns geo-targeted 15- and 30-second pre-roll ad inventory in those states, other campaigns are still able to bid on other forms of YouTube ads through the auction system.

If Romney has staying power as many predict, it should result in far more money going to ad networks than directly to publisher sites. Romney’s digital team, headed by Zac Moffatt and assisted by digital ad agency Targeted Victory, relies heavily on intricate data-driven targeting. In many cases, the idea is not just to target Republicans, but to aim ads at Republicans with a higher propensity to voting for Romney.

The campaign also works with Say Media to target video ads to non-TV watchers whom television spots are bound to miss. The non-direct approach also allows for speedier changes to ad creative, when waiting for a publisher site to switch up creative might take too long. Another consideration for the campaign is cost: In many cases the cost is lower when buying inventory through networks as opposed to direct from publishers. Especially when it comes to pre-roll video advertising, other political advertising sources suggest local news sites are charging too much this cycle, despite the fact that many media buyers can purchase the same inventory through ad networks at lower costs, thus circumventing cost-prohibitive direct buys.