PHOENIX – Arizona lawmakers expressing doubt over whether President Obama was born in the United States are pushing a bill through the Legislature that would require the president to show his birth certificate to get on the state’s 2012 ballot.

The House passed the measure on a 31-29 vote, ignoring protests from opponents who said it’s casting Arizona in an ugly light and could give the elected secretary of state broad powers to kick a presidential candidate off the ballot.

“We’re becoming a national joke,” Rep. Chad Campbell, a Phoenix Democrat who opposes the measure, said Thursday.

The measure’s sponsor, Republican Rep. Judy Burges of Skull Valley, said she isn’t sure Obama could prove his eligibility for the ballot in Arizona and wants to erase all doubts. “You have half the population who thinks everything is fine, and you have the other half who has had doubts built up in their mind,” Burges said.

So-called “birthers” have contended since the 2008 campaign that Obama is ineligible to be president because, they argue, he was actually born in Kenya, his father’s homeland. The Constitution says a person must be a “natural-born citizen” to be eligible for the presidency.

Hawaii officials have repeatedly confirmed Obama’s citizenship, and his Hawaiian birth certificate has been made public, along with birth notices from newspapers published within days of his birth in August 1961.

Courts have rebuffed lawsuits challenging Obama’s eligibility.