You have to have an paid account to see the actual paper so all I have to go on is a report in Huffington.
This study investigated biased message processing of political satire in The Colbert Report and the influence of political ideology on perceptions of Stephen Colbert. Results indicate that political ideology influences biased processing of ambiguous political messages and source in late-night comedy. Using data from an experiment (N = 332), we found that individual-level political ideology significantly predicted perceptions of Colbert's political ideology. Additionally, there was no significant difference between the groups in thinking Colbert was funny, but conservatives were more likely to report that Colbert only pretends to be joking and genuinely meant what he said while liberals were more likely to report that Colbert used satire and was not serious when offering political statements. Conservatism also significantly predicted perceptions that Colbert disliked liberalism. Finally, a post hoc analysis revealed that perceptions of Colbert's political opinions fully mediated the relationship between political ideology and individual-level opinion.
Notice that the study didn't say where Colbert is on the political spectrum, but Jason Linkins who wrote the post knows that Colbert is a liberal and that conservatives are outright stupid to believe otherwise. That actually matches up with the OSU study - liberal think that Colbert is liberal. One of my conservative co-workers is convinced that Colbert is a secret conservative so that matches up, also.
So what is he? Short answer - he is a political comedian playing a role. Most of his lines are written with the help of a staff of writers and the writers are to the left of the real Colbert. Colbert follows the old rule, don't cut funny, so left-leaning jokes stay in if they are funny.
How do I know that he is not as liberal as his writers? Because I watch the show. More important, I watched it before, during, and after the writers strike. Unless Colbert had a staff of right-wing ghost-writers working for him during the strike, those episodes were examples of his real views and they were considerably more moderate than his show with writers.
Then there is his religion. On the show he is an over-the-top Roman Catholic. If you assume that he is the opposite of his character then you have trouble reconciling the fact that he is a practicing Catholic who attends mass regularly.
The real proof is in the interviews. Colbert has the reputation as one of the toughest interviewers on tv. He actually does three different types of interviews. When he is interviewing scientists or musicians then he is a pretty easy interview. When his guest is a conservative then he makes them work. He really turns mean when he interviews liberals. He doesn't let them get their message out. He interupts them with questions like, "George Bush, great president or greatest president?" His "Better know a district" series of interviews always makes the recipient look foolish but more often than not, he is interviewing Democrats. Would a true-believer liberal go out of his way to make Democrats look bad?
During the 2008 campaign, the two candidates who got the friendliest treatment from Colbert were Dennis Kucinich and Mike Huckabee. Satire or not, I just can't imagine Jon Stewart playing foosball with Huckabee for the primary.
Like his religion, Colbert's real feelings are probably far more moderate than his fans on the left would believe. The secret of his success is that his show is not based on his beliefs. That frees him to make fun of both sides.
I suspect that each side sees something that they find funny and convinces themselves that Colbert represents their own views. The left, since they consider themselves smarter than everyone else, then laughs at the right for being fooled by Colbert. The joke is probably as much on them as on the right but they are too busy looking down their nose to see what it under it.