CRJ700 Captain reports TCAS RA with light aircraft while on a visual approach to Runway 30 at LGB. Evasive action is taken but reporter is able to regain the glide slope and land.

Narrative

We were vectored for an approach to Runway 12 at LGB. The weather was VFR with a layer of clouds obscuring the northern part of the airport and approach. We reported that we would not be able to accept a visual for 12 at LGB after getting to a position where we should be able to make a normal VFR approach to the field. ATC vectored us back across the airport to take us down wind to set us up for the ILS approach. As we crossed the airport heading southwest; the airport came into sight for a perfect set up for a left base visual to 30. I asked for the visual and we completed the landing checklist about 3 miles out; the Tower called out traffic to us (a small aircraft) was crossing our final approach. We could not get the smaller aircraft in sight and received an RA for which I immediately maneuvered and started a climb. Gaining only 2-3 hundred FT before clear of the conflict; I was able to regain the glide path and resume a normal landing. Other than the scattered cloud layer that obscured the northern half of the airport; the weather was generally good VFR 10 miles visibility. The LGB airport is one of our most demanding airports traffic wise. These traffic conflict irregularities occur constantly in that approach airspace and within the Class D airspace. I believe the Tower Controllers should do a much better job.