My colleagues are saying Donald Trump won the debate, because in his absence the rest of the Republican candidates cut each other down. But Trump won the debate in another way: His absence from the debate appears to have hurt viewership, as he predicted.

According to Nielsen, about 12.5 million viewers watched the Fox News debate, which is about half of the audience Trump drew to the first Fox News debate in August, when the event drew a record-breaking 23 million viewers.

The chart above might make it seem like last night's viewership wasn't terrible, especially compared with the previous GOP debate on the Fox Business Network. But given that this debate was on Fox News, which has the highest primetime viewership of any cable news network, this has to be a disappointment to both the network and the candidates hoping to make one last push before the Iowa caucus.

Why Trump skipped the debate

Trump is leading in most polls, so, at the least, he can benefit from low debate viewership. If Ted Cruz or Marco Rubio had made a strong showing but fewer viewers saw it, then all the better for the Donald.

At the same time, Trump was able to control the news cycle with his public beef with Fox News, as well as the counterprogramming with his veterans fundraiser. Evening news shows spent the large majority of their time talking about Trump's absence rather than the actual debate.

The winner in a Trump-less debate is ... Ted Cruz?

The ironic part is that if this debate somehow lowers the buzz and enthusiasm around the primary and keeps caucus turnout low, it actually benefits Cruz, who is polling second behind Trump in Iowa. To be clear, I'm not saying low viewership is always a harbinger of voter enthusiasm — and given the amount of coverage there has been around Trump's beef with Fox News and Megyn Kelly, it's even possible more people have been engaged with this debate despite not watching.

But for Trump, it's important for his supporters to stay engaged. According to Monmouth University pollsters, a Trump victory depends on "lone wolf caucusgoers" who are haven't caucused before — and the polls actually suggest these people are going to show up. The question is how many of them?

Democratic debates also lack Trump — and also get low viewership

For Republicans, the interest Trump has brought to the election is undeniable — and that makes it easier for him to sit out events like the Thursday debate. About 86 percent of Republicans say they are giving some or a lot of thought to this election, compared with 74 percent during the 2008 primary season. In addition, 81 percent of Republicans say they are very or somewhat enthusiastic about this election, versus 64 percent of Democrats.

For Democrats, Trump's ability to draw huge debate audiences is something they are self-conscious of. The Democratic National Committee only scheduled six debates — and half of them on days people generally don't watch — which has drawn criticism from many within the party, especially with comparatively low viewership numbers in debates thus far. But several Democratic operatives have told me that comparing Democratic debate viewership numbers with Republican ones has to be done in the context of the "Trump effect."