Mr. Reid’s re-election fight would have been a high-spending slugfest in the presidential battleground of Nevada. Conservatives such as Charles G. and David H. Koch, the billionaire brothers who were a favorite target of Mr. Reid’s criticism in 2014, would most likely have spared no expense in trying to oust him.

Now the race for the open seat will figure prominently in the struggle for control of the Senate. Catherine Cortez Masto, a former state attorney general, is considered a leading Democratic candidate with Mr. Reid out; the Republican field will be fluid and is likely to include Michael Roberson, a State Senate leader.

Mr. Schumer, who helped Democrats capture the Senate in 2006 and has been Mr. Reid’s right-hand man in political messaging, is considered the favorite to replace Mr. Reid, who later on Friday endorsed Mr. Schumer for the leadership position and said he believed Mr. Schumer would receive the necessary support.

Mr. Reid had previously insisted he was running and said he was confident that he could have triumphed next year had he decided to seek a sixth term. A onetime amateur boxer, he said that he might not have even run in 2010 if Republicans had not made such a point of trying to unseat him.

He also said he was worried his race would consume campaign money that would be needed in other competitive states as Democrats try to regain control of the Senate.

“I think it is unfair for me to be soaking up all the money to be re-elected with what we are doing in Maryland, in Pennsylvania, in Missouri, in Florida,” he said. “These are big, expensive states.”