I don't think they would "volunteer" to go home and wait in line; that would be highly uncharacteristic for these fence-jumpers. On the other hand, if the Federal and state governments started cracking down on employers who hire illegals, if states cut off all of the benefits, and there was no longer any "anchor baby" provision, it would certainly remove a lot of incentives for illegals to stay in the country. The only recourse for many of them would be to leave.
We have over 40 million Hispanics in the US. According to the 2000 Census, there are nearly 9 million Mexican-born residents with 4 million in California alone. People would rather stay than go back to the hell hole they left at great risk and expense. They have a support network in the US, which still offers them more hope and opportunity than returning to their home countries. And don't kid yourself, undocumented workers will still be attractive to employers, including individual citizens. They will be even more so if the number declines.
Others may turn to crime and the drug trade joining a significant number of their compatriots. The bottom line is once you get here, you want to stay.
I don't think people appreciate the massive bureaucratic resources it would take to enforce the laws we have now, let alone what would be needed to implement McCain-Feingold. Background checks on up to 20 million illegal aliens, fines, employer sanctions, etc. First, we need to cut off the supply, i.e., secure the borders, then worry about how to handle who is here.