By Alan Silverleib, CNN Congressional Producer
Washington (CNN) -- The first congressional votes were cast on Thursday on the politically explosive issue of immigration reform.
Members of the Democratic-controlled Senate Judiciary Committee started formal consideration of a massive bipartisan bill, debating and voting on the first of more than 300 proposed amendments.
The contentious issue of border security was quickly raised in the form of a proposed "trigger" amendment from Iowa's Chuck Grassley -- the top Republican on the panel. It would block the legalization of any undocumented residents until law enforcement established "effective" control of the entire U.S.-Mexico border for six months.
Grassley's proposal failed, with 12 of the panel's 18 members voting no. South Carolina Sen. Lindsey Graham and Arizona Sen. Jeff Flake -- two Republicans from the bipartisan "Gang of Eight" group that crafted the overall bill -- joined committee Democrats in opposing the measure.
The four Democrats and four Republicans comprising the "Gang of Eight" have pledged to oppose any substantial changes to the legislation.