Home come Issa's race is so close this year?
Combination of factors:
1. Lots of College-educated voters in this district, one of Trump's worst demographics
2. Issa is one of Trump's biggest supporters in Congress
3. Most of the district is in north San Diego County. Republicans have a registration advantage there, but it isn't as solid as previous elections
4. Related to #3, 'County Club Republicans' are gradually being replaced by a younger & more diverse population
5. Formidable Democratic opponent, an Iraq War vet in a district with a major Marine base
6. DCCC started pouring money earlier this month & running TV ads against Issa