Sens. Bernie Sanders (I-TV), Patrick Leahy (D-VT), Richard Durbin (D-IL) and Bill Nelson (D-FL) on Tuesday requested that the Government Accountability Office investigate the impact of new state laws aimed at thwarting voter fraud.

"It is critical that we have an accurate picture of these recent state laws, individual access to voting, and actual instances of voter impersonation fraud," the senators wrote in a letter to the GAO, the non-partisan research arm of Congress.

Republicans across the country have pushed for stricter voting regulations, such as voter ID laws, to protect against alleged voter fraud. More than 30 states have changed voter laws since 2008, including requiring voter identification cards, eliminating same-day registration on voting day, prohibiting ex-felons from ballot access, restricting early voting and requiring proof of citizenship.

But a report by New York University Law School’s Brennan Center for Justice found that changes to voting laws could suppress up to five million votes during the 2012 elections, particularly among young, minority and low-income voters, as well as those with disabilities.

"While instances of voter impersonation fraud -- the purported justification for these unnecessarily burdensome requirements -- are reportedly minimal, these legislative changes impose costs and resource requirements on those seeking to exercise their constitutional right to vote that effectively deny it," the senators wrote.

"State actions that suppress the right to vote must not be tolerated. We must make it easier, not harder, for poor and working people to vote and to participate in the political process."