If voting by illegal aliens is not a problem, why are "immigrant advocates" so vehemently opposed to people having to show proof of citizenship when voting?

From "Critics blast Horn's bid to reduce fraud":

Californians would have to show proof of citizenship when registering to vote and photo identification when voting under a proposal San Diego County Supervisor Bill Horn is pushing.

Horn, who said the requirements would help prevent voter fraud, will ask the Board of Supervisors tomorrow to sign a letter to the secretary of state endorsing his proposal and advocating a change in state law.

Civil rights advocates, the San Diego County Democratic Party and the San Diego League of Women Voters say the proposed requirements are burdensome and would discourage people — especially minorities — from voting. The Legislature has rejected similar recommendations before, they said.

"We have not seen a rash of voter fraud taking place in California, certainly not in San Diego County, that I've been made aware of," said Jess Durfee, chairman of the county's Democratic Party...

From "Voting by noncitizens hard to find":

Concern that illegal immigrants vote regularly in Arizona has become an emotional hot button for proponents of Proposition 200, but election officials say there is little or no evidence that voting by noncitizens is a problem.

Those officials, including Republican Secretary of State Jan Brewer, say they know of no cases in which a noncitizen was convicted and few if any instances in which anyone was prosecuted for voting illegally. Pima County Recorder F. Ann Rodriguez, in fact, says the illegal-immigrant voter is an urban legend...

Here's a tonier allusion than Milhouse: methinks they doth protest too much.