President Barack Obama's nomination of Maryland Labor Secretary Thomas E. Perez to a top Justice department position has officially been sent to the Senate, the White House press secretary's office announced Tuesday afternoon.

Perez, who is Obama's pick to be the nation's top civil rights enforcer, faces likely opposition from conservatives, who have already begun criticizing his past positions on issues such as affirmative action.

Conservative columnist Linda Chavez, the unsuccessful Republican nominee for U.S. Senate in Maryland against Democratic Sen. Barbara A. Mikulski in 1986, recently took aim at Perez over race-based medical school admissions. Chavez criticized a 2006 law review article by Perez for the University of Maryland's Journal of Health Care Law and Policy in which, she said, he argued for "explicit race-conscious admissions policies for medical school."

Perez has yet to begin making rounds of courtesy calls on members of the Senate, who are to consider his nomination as assistant Attorney General for civil rights, according to a spokeswoman for Democratic Sen. Benjamin L. Cardin of Maryland.

Cardin, a member of the Judiciary committee, hopes to chair Perez's confirmation hearing later this spring. No date has been set yet.