For all the griping about media bias in politics, good data is in short supply. Every four years, however, the nonpartisan Pew Research Center releases exhaustive, quantitative reports on how the press covers the presidential campaign. Their new report, out Monday, shows that the largest bias this year did not favor an ideology or candidate—though Santorum never got much love—but favored the coverage of the horserace and personal issues over public policy.

The press covered the horserace seven times more than domestic issues in the GOP primary. That made it the most covered topic by far, as Pew reports in this chart:

While it’s hard to see what voters are supposed to base their decisions on if most coverage is about tactics, not the actual issues in the race, Pew notes that 2012 was actually better on this score than last cycle. Then, strategy made up a whopping 80 percent of press coverage about the GOP field, and 78 percent for the Democrats. That may have been because the 2008 race had even more drama between the candidates.

As for the overall tone of candidate coverage, Pew found that Romney received primarily positive press once he established himself as a likely nominee (after the Michigan primary), which reflects the emphasis on the horse race. (Leading candidates get better coverage, regardless of their views, when the race is covered primarily as a game.) The exception was Santorum, who got mostly negative coverage even as he was the second most successful competitor.

“The former Pennsylvania senator was never able to sustain substantially positive coverage for more than two weeks,” explains the report, “and often not more than one.” Santorum did get a boost in overall coverage during his victories, and he drew the majority of coverage as a lead newsmaker through much of February and March.

There was one candidate who drew even more negative coverage than Santorum. It was not Newt Gingrich, who frequently complained about a raw deal from the press. It was not Ron Paul, whose loyal constituency was often underplayed, especially on the influential Fox News channel. No, according to the data, it was that other guy. “Of all the presidential candidates studied in this report, only one figure did not have a single week in 2012 when positive coverage exceeded negative coverage—the incumbent, Democrat Barack Obama,” reports Pew.  Here’s the chart over time:

No one talks about an “anti-incumbent” bias. But actually being in charge tends to invite plenty of criticism.