Yet given his own deep political ambitions and drive, Mr. Schumer is also very carefully laying the groundwork to move on a moment’s notice to try to secure the top Democratic job if Mr. Reid is defeated on Tuesday.

The situation for Mr. Schumer is further complicated by the fact that his partner in the Senate leadership and Capitol Hill housemate, Senator Richard J. Durbin of Illinois, is also positioning himself in the underground campaign to replace Mr. Reid, though both men have made it clear they would greatly prefer that Mr. Reid return. The senators and their aides declined to comment for this article.

Mr. Schumer, one of the party’s most prolific fund-raisers, has distributed $4 million from his own campaign coffers to the party and his colleagues this election cycle, including the $500,000 turned over to the Nevada Democratic Party to help Mr. Reid. Mr. Durbin has given nearly $500,000 to the campaign effort of Senate Democrats, and has appeared at fund-raisers and events and barnstormed with Mr. Reid.

Neither Mr. Schumer nor Mr. Durbin appears to be openly courting support for the job. And in the treacherous world of Senate leadership elections, even a hint of acting before Mr. Reid’s fate is known could be costly.

“With all of the problems our party is having this cycle, he has really put the kibosh on any conversations about Harry Reid other than we all have to support him,” Senator Claire McCaskill, Democrat of Missouri, said of Mr. Schumer. “I think everyone has respected Chuck’s leadership in that regard.”