WASHINGTON—The Senate narrowly confirmed Todd Blanche as attorney general, cementing his place atop the Justice Department after a contentious battle that focused on his willingness to bend the agency to President Trump’s political and financial interests.The Senate narrowly confirmed Todd Blanche as attorney general,Blanche was confirmed Saturday by a 50-49 vote, with Republican Sens. Lisa Murkowski of Alaska and Susan Collins of Maine joining the Democratic minority in voting against him.For Blanche, the confirmation adds prestige rather than power, because he has been acting as attorney general in a temporary capacity since April while also serving as the Justice Department’s No. 2 official.The vote, taken just before senators’ August recess, capped one of the most unusual confirmation processes for attorney general in recent memory. The episode was also indicative of the kinds of political challenges Blanche will face as a Senate-confirmed attorney general who finds himself between a mercurial president and a legal system increasingly skeptical of the Justice Department’s credibility.“It is not as if I remain blissfully unaware of what will happen if I am confirmed,” Blanche said at a recent press conference.With Blanche’s confirmation, Trump secures a trusted ally atop the Justice Department. Blanche, 52, a former federal prosecutor in Manhattan who later became a defense attorney, helped Trump navigate several criminal cases, including a pair brought by special counsel Jack Smith during the Biden-era Justice Department. Since Trump fired his predecessor, Pam Bondi, in April, Blanche has moved with speed and decisiveness, advancing investigations and prosecutions of the president’s favored targets, including former FBI Director James Comey.Blanche’s fate was uncertain until late Friday morning, when Sen. Bill Cassidy (R., La.) announced he would provide the pivotal yes vote. “I’ll be criticized for this vote. What’s new?” Cassidy said, appearing to grow emotional during a speech on the Senate floor. “But the people of Louisiana can be assured that I worked hard to understand the issues and make the right decision.”In opposing Blanche, Sens. Lisa Murkowski (R., Alaska) and Susan Collins (R., Maine) cited concerns about what they viewed as a politicized Justice Department, and statements in which Blanche suggested he supported prohibitions on the mailing of abortion medication.Blanche had antagonized some Republican senators with his support for a much-criticized deal to resolve Trump’s personal lawsuit against the Internal Revenue Service: a $1.8 billion “anti-weaponization” fund to compensate his allies and immunity from IRS audits for past tax returns.For weeks Blanche was forced to negotiate with Republican holdouts, Sens. John Cornyn of Texas and Thom Tillis of North Carolina, who sought Blanche’s written assurances that the fund was dead and the tax-audit protections limited, in exchange for advancing his nomination through the Senate Judiciary Committee.The result was a pair of documents that appeased the two senators but, legal experts said, left the door open to the resurrection of at least portions of the settlement. Blanche narrowed the scope of the tax immunity but it still grants Trump what could be a significant gain; Democrats are introducing legislation to kill those benefits.Democrats say Blanche still views himself more as Trump’s defender than an apolitical law enforcer with an eye toward the agency’s historic independence from the White House.He also hasn’t been able to escape bipartisan criticism of the department’s bumpy release of investigative files related to convicted sex offender Jeffrey Epstein. Tillis demanded Blanche meet with some of the disgraced financier’s victims and their families, which he did at the Justice Department last month.Write to Sadie Gurman at sadie.gurman@wsj.com and Lindsay Wise at lindsay.wise@wsj.com