Google's full employment program for antitrust regulators continues: the search giant is in the final stages of a deal to purchase ad tech company AdMeld. Like other recent Google purchases, this deal will automatically generate scrutiny from Washington before it can formally close.

That's both because of the size of the deal--around $400 million--and because the purchase deals with a sector that Google already dominates--display ad sales.

AdMeld is one of a handful of big ad optimization platforms that work on behalf of publishers by trying to get the best prices for their inventory from a variety of ad networks. That kind of work is one of the few parts of the ad tech ecosystem where Google didn't already have a presence, so a deal for AdMeld or one of its peers always seemed inevitable.

Google declined to comment; AdMeld CEO Michael Barrett hasn't responded to requests for comment.

Google's display ad team, led by Neal Mohan, has spent the last couple years expanding its footprint in the ad tech ecosystem. In 2009, it bought Teracent, which customizes online ads on the fly. A year ago, it bought Invite Media, which helps ad buyers place orders for ad inventory via "real time" ad exchanges. And all of this stuff hovers around its DoubleClick ad exchange platform, which it bought in 2007 and relaunched last year.

Here's one way to think about the ad tech landscape: a chart created by Luma Partners banker Terry Kawaja that shows the dizzying number of companies that have cropped up in recent years. Don't worry if you can't understand it--it's designed in part to be a bit obtuse and intimidating. What's important to recognize here is that up until now Google has had a presence on the left side and center of the chart; if the AdMeld deal goes through, it will extend its reach to the right side, too.

[UPDATE: Thanks to the several readers that pointed out that I don't know how to read a chart, and more important, that I've understated Google's existing relationship with publishers: Google's DFP platform already serves ads for publishers, and the DoubleClick exchange sells ads on their behalf, too. A better way of making the point: the AdMeld deal will give Google even more ways to do business with publishers.]

Will Washington sign off on this one? Google obviously thinks so, and the fact that Google got the go-ahead for an even larger deal last year--its $750 million AdMob purchase--should give it confidence.

One difference, though: last year, Google successfully argued that buying AdMob wouldn't make it dominant in the mobile advertising industry because the mobile advertising industry barely existed.

Displays ads, however, are a real business, in the single-digit billions. That number will grow much bigger--Google Chairman Eric Schmidt has thrown out a $200 billion goal--but right now Google, via DoubleClick, has a big chunk of what's there. Earlier today, Mohan gave a speech at an industry conference outlining his vision for the market; you can see a summary here.

AdMeld's two primary competitors have been Rubicon and Pubmatic. Last year News Corp., which also owns this Web site, merged its display ad technology assets with Rubicon.

TechCrunch first reported news of the Google AdMeld deal.