Redlands mayor Pete Aguilar's 2012 CA-31 candidacy ended up being the Democratic most embarrassing moment of last year's campaign. He was the DCCC handpicked candidate in a district that favored Democrats. He was the favorite to win the seat. Under California's new primary system, the top two finishers make the general election and the top two were both Republicans. Aguilar finished third and Democrats didn't get to compete for the seat.
Aguilar certainly failed in 2012 and became a joke punchline. It wasn't all his fault. He's correct that one of his problems was getting into the race so late. Two of his opponents had a six month jump on him and had already secured a base of votes when he got in. Justin Kim got in the race late, but he managed to rally Asians around his candidacy. Aguilar failed to work hard enough to take voters away from the other three candidates.
If there aren't other Democrats, or even if there's only one, he'll make top two without putting in any more effort than he did in 2012. He'll make the general election and then we'll see if the district is one a Republican can win one-on-one. The district went 57.2% Barack Obama in 2012 and no Republican in the country won a district where President Obama got more than 54.6%. That'd seem to make the district a slam dunk for Democrats. There are two factors to keep in mind.
1) This'll be a mid-term year, with an electorate closer to 2010 than 2012. Republican congressional candidates narrowly won the district in 2010 if you sum the votes of all the district's voters by party. Steve Cooley won fairly comfortably in the Attorney General race and Carly Fiorina narrowly lost the senate race.
2) Republican Gary Miller is an incumbent.
Starting early should help Aguilar.