Senator Bernie Sanders, the progressive Vermont independent, has emerged as a contender for labor secretary in President-elect Joseph R. Biden Jr.’s administration, a prospect that would suit his ambitions of being a warrior for working Americans — and one that makes some Senate Republicans very uneasy.

“I think that is somebody who we know is an ideologue and, well, it would be very unlikely he would be confirmed in a Republican-held Senate,” said Senator John Cornyn of Texas, one of multiple Republicans who said Mr. Sanders, a self-described democratic socialist, would be unlikely to win the chamber’s approval.

It is a testament to the deterioration of the Senate confirmation process that a longtime colleague — even one they vehemently oppose on policy — would face such a Republican roadblock. In the not-too-distant past, fellow senators got considerable leeway from the opposing party if they were selected to join the executive branch.

“The truth is, to the best of my knowledge, there has been a courtesy within the Senate that when a president nominates senators, they have been approved,” Mr. Sanders said in an interview.

The growing senatorial resistance to Mr. Sanders even before any formal action by the new administration reflects the formidable task Mr. Biden faces. Should Republicans hold on to their Senate majority next year, Mr. Biden would be the first president since George Bush in 1989 to enter office without his party controlling the chamber and managing the confirmation process. And that process has grown much more toxic, to the point where senators routinely engage in near-blanket opposition to the picks of a president from the opposite party — if they allow consideration at all.

“It is sort of uncharted waters,” said Lindsay M. Chervinsky, a presidential historian and the author of a book on George Washington’s cabinet. “For most of history, the Senate has given presidents, especially first-term presidents, wide berth. They usually give the president who they want.”

That is no certainty today. Some Republicans, who need to win at least one of two Senate runoff elections in Georgia on Jan. 5 to hold their slim majority, have already made clear that they are not eager to grant much latitude to Mr. Biden when it comes to nominees. They note efforts by Democrats over the past four years to block President Trump’s picks and to force Republicans to clear every time-consuming procedural hurdle even when the final outcome was inevitable.

“I can assure you that there will not be one set of rules for Donald Trump and, should Joe Biden take office, another set of rules for him,” Senator Tom Cotton, Republican of Arkansas, said this week on the radio show of Hugh Hewitt, a conservative host. “What the Democrats have done for the last four years, if it is good for the goose, it is going to be good for the gander as well.”

Other Republicans — including Senators Susan Collins of Maine, Lindsey Graham of South Carolina and Lisa Murkowski of Alaska — have indicated they would be willing to back Mr. Biden’s picks as long as they were considered mainstream, acknowledging that a Democratic president is entitled to selections that comport with his views.

They and other Republicans say potential candidates who could meet that test include Senator Doug Jones, the Alabama Democrat who lost his re-election bid this month; Antony Blinken, a longtime Biden foreign policy adviser; and Senator Chris Coons, Democrat of Delaware and a Biden confidant.

But the overall attitude appears to be one of skepticism, and Republican senators have signaled that rather than feeling an onus to give the president his team, they believe it is squarely on Mr. Biden to find nominees who can pass muster with them. Besides Mr. Sanders, Republicans have also indicated that they would disapprove of nominations of Senator Elizabeth Warren, Democrat of Massachusetts, and Susan Rice, the former national security adviser to President Barack Obama.

Not just the top slots are at issue. The Senate must also confirm scores of other executive branch nominees to lower-level posts that are critical to running sprawling agencies.

If Republicans hold on, it will be up to Senator Mitch McConnell of Kentucky, the majority leader who refused to even consider Mr. Obama’s Supreme Court nominee in 2016, to put nominations on the floor. He is not one to take action that does not have widespread backing from Republican senators and party voters. (Text, excluding headline, courtesy New York Times)

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