Kerry had a similar issue available to him, but he didn't use it: Bush's guest worker plan. That plan isn't just for field workers: it would be "open-ended", open to the world, and it could drive wages for higher-wage workers down near the minimum wage.
I'm sure there are a lot of Republicans/conservatives who'd like to see the Dems take on illegal immigration. Bush is tremendously weak on this issue, and he's lost some votes and money because of it. See my immigration categories if you'd like background information on this issue.
I'd also suggest looking at the Prop. 200 (AZ) exit poll and how close Cynthia Matthews got to David Dreier because of Political Human Sacrifice.
On the topic of gun control, a great number of people feel that a disarmed populace is an invitation to tyranny. That's certainly not a black helicopter viewpoint, but I can't prevent those who have no sense of history from thinking it is.
Also, Dreier's opponent apparently got little or no money from the Dem party. Perhaps it was because she came out against illegal immigration, or perhaps it was because there's a bit of a gentleman's agreement involved not to unseat people like Dreier.
Summary: two SoCal talk jocks (John & Ken @ KFI) encouraged their listeners to vote against Dreier due to his support for illegal immigration. Many Republicans just couldn't bring themselves to vote for a Democrat, and no doubt the uptick in L votes was because Republicans wanted to vote against Dreier but couldn't vote Democrat.
On a somewhat related note, see Why Kerry won, where I discuss the issues he focused on that lead to his landslide.