The only thing I can see that makes Trump a "clear favorite" at this stage is incumbency and the fact that so far he's the only candidate who is for sure on the ballot (unless RNC pulls their support which I don't see happening). Other than that, to call him an early favorite now is like saying he had no chance of winning in 2015. There aren't any facts to really back that.
Some things we know:
- There is no indication that Trump has built a greater coalition than what he had last election.
- A few of the Senate Republicans Trump campaigned for flipped some seats. Granted they were all in red states, but it's noteworthy in a blue wave year. On the other hand, some races were closer than they should've been (Texas) and Arizona isn't reliably red anymore.
- The "blue wall" that seemed to break for Trump in 2016 seemed to trend toward Democrats again in 2018, suggesting 2016 could've been a fluke. This is Trump's best, and maybe only, path toward victory. He only won these states by small margins and has little room for error.
- Hillary Clinton was a bad, unpopular candidate. So far it doesn't look like she'll be running again.
In the meantime, we have to wait and see who the Democrats nominate, what the economy looks like, and what the divided government does this year. So far the only thing of note is the government shutdown, which didn't go well for Trump.
There are also investigations that might not lead to his removal from office but can do damage but probably aren't going to be his biggest problem unless something really bad comes out. I don't think anything he does foreign policy-wise will make an electoral difference.