WASHINGTON — Federal prosecutor Loretta Lynch won confirmation to serve as attorney general Thursday from a Senate that forced her to wait more than five months for the title and remained divided to the end.

The 56-43 vote installs Lynch, 55, now U.S. attorney for the Eastern District of New York, as the first black woman in the nation’s top law enforcement post. She will replace Eric Holder, a perennial lightning rod for conservatives who was once held in contempt of Congress.

The vote total for Lynch was the lowest for any attorney general since Michael Mukasey won confirmation with 53 votes in 2007 after Democrats decried his refusal to describe waterboarding as torture.

For Lynch, the issue that tore into her support with Republicans was immigration and her refusal to denounce President Barack Obama’s executive actions limiting deportations for millions of people living illegally in this country. Questioned on the issue at her confirmation hearing in January, she said that she believed Obama’s actions were reasonable and lawful.

Democrats angrily criticized Republicans for using the issue against her, but Republicans were unapologetic.

Announced GOP presidential candidate Sen. Ted Cruz, R-Texas, said Lynch’s comments rendered her “unsuitable for confirmation as attorney general of the United States. That was a shame.”

Yet after returning from the campaign trail to rail against Lynch on the Senate floor, Cruz was the only senator absent when the vote was called.

Still, Lynch won the support of 10 Republicans, more than expected in the days heading into the vote. In a surprise, Senate Majority Leader Mitch McConnell, R-Ky., was among those voting “yes.”

The long delay in confirming Lynch since she was nominated in November incensed Democrats, with Obama himself weighing in last week to lament Senate dysfunction and decry the wait as “crazy” and “embarrassing.” There were various reasons for the delay, most recently a lengthy and unexpected impasse over abortion on an unrelated bill to combat sex trafficking.

Yet Democrats controlled the Senate when Lynch was nominated last November and could have brought up her nomination for a vote then. They held off with the GOP’s encouragement after being routed in the midterm elections, and spent the time confirming judges instead.

There was an expectation that Republican leaders would move Lynch’s nomination swiftly this year, especially since most GOP members of Congress loathe Holder, who’s seen as too politically close to Obama and even more liberal. But instead, the nomination became tangled in the dispute over Obama’s executive actions on immigration, and seemed to stall.

Lynch, who grew up in North Carolina, has been the top prosecutor since 2010 for a district that includes Brooklyn, Queens, Staten Island and Long Island, a role she also held from 1999 to 2001.