In July, President Biden announced that he intended to nominate Deborah E. Lipstadt, a renowned Holocaust scholar, to lead a new office at the State Department assigned to battle soaring antisemitism around the globe.

The decision drew praise from more than 20 liberal and conservative Jewish groups, all of whom were impressed with Dr. Lipstadt’s sterling credentials and her reputation for standing up to antisemitism wherever she saw it, whether it was neo-Nazi marches in Charlottesville, Va., or a liberal icon in Congress.

Yet nearly six months later, Dr. Lipstadt’s nomination remains in limbo, thwarted by Senate Republicans who have complained that she criticized some of them on Twitter.

Dr. Lipstadt is among the most prominent of hundreds of Biden nominees whose bids for Senate-confirmed jobs have languished because of partisan dysfunction or personal pique. In a rare though hardly shining example of comity, members of both parties agree the confirmation system is a contentious mess, owing in part to what Senator Mitch McConnell, Republican of Kentucky and the minority leader, has called “turf problems.”

The problem appears to be the worst it has ever been. A year after Mr. Biden’s inauguration, only 41 percent of his nominees for Senate-confirmed posts have been approved, according to a new analysis by the Partnership for Public Service, a nonpartisan group that seeks to make the federal government more effective.

Mr. Biden, for his part, has issued nominations at a faster pace than President Donald J. Trump did, but slower than Presidents Barack Obama and George W. Bush, according to the analysis. Regardless, it has taken an average of 103 days for the Senate to confirm Mr. Biden’s nominees — about a month longer than in the Obama administration, about twice as long as in the Clinton administration and nearly three times as long as during the Reagan era.

“You’re seeing a broken system breaking down even further, and in an election year it’s not going to get better,” said Max Stier, the Partnership’s chief executive. “We need a political Geneva Convention, to distinguish between legitimate partisan differences and the destruction of our core government infrastructure.”

Late last month, Senator Chuck Schumer, Democrat of New York and the majority leader, agreed to schedule a potentially contentious vote on imposing sanctions on the company behind a Russian-laid natural gas pipeline to Germany to satisfy Senator Ted Cruz, Republican of Texas, who had blocked scores of State Department nominees over the issue. Soon afterward nearly 40 nominations cleared the Senate, including Mr. Biden’s picks to be the U.S. ambassadors to China and Japan. But scores of others remain stuck.

“The truth is that some Republicans’ unprecedented obstructionism is straining the system to the breaking point,” Senator Bob Menendez, Democrat of New Jersey and the chairman of the Foreign Relations Committee, said on the Senate floor last month, adding that the situation was forcing the president to operate without critical national security officials in place, “leaving our nation weakened.”

Charts supplied by a staff member for the committee’s top Republican, Senator Jim Risch of Idaho, suggested the committee was moving faster on nominations than in the previous Congress, when Mr. Risch was the committee’s chairman.

But more than 15 other Senate committees have jurisdiction over some nominations. And the foot dragging extends beyond blocking committee hearings on nominees.

Last month, Senator Tom Cotton, Republican of Arkansas, briefly refused to confirm five U.S. attorney nominees from Democratic-leaning states, demanding on the Senate floor that Senator Richard J. Durbin, Democrat of Illinois, first apologize for interrupting him more than eight months earlier during a hearing. The Senate voted to confirm all five nominees soon after Mr. Durbin apologized.

This month, the White House resubmitted more than 100 nominations after the Senate adjourned for the December recess without taking action on them. Some of those nominees have been waiting nearly a year to begin work, including Dilawar Syed, who was originally nominated in March as deputy administrator of the Small Business Administration. Republicans’ stated objections to confirming Mr. Syed, who would be the highest-ranking Muslim in the federal government, include his work for a Muslim advocacy group. But they also have cited their opposition to the Small Business Administration’s decision to approve pandemic aid to abortion providers.

Mr. Biden also renominated Ed Gonzalez, the sheriff for Harris County, Texas, to lead Immigration and Customs Enforcement, after originally nominating him in April. Despite its critical role in controlling the flow of immigrants over the southern border, ICE has not had a permanent leader since 2017.

Into this maelstrom went Dr. Lipstadt’s nomination.

The White House announced in late July that Dr. Lipstadt would lead an expanded office at the State Department focused on tracking and countering the rise of antisemitism abroad. For the first time, the role would carry the rank of ambassador, requiring Senate confirmation.

Mr. Risch declined last month to say when Republicans would consent to a hearing on Dr. Lipstadt’s nomination. Mr. Risch and other Republicans have alluded to the holdup being tied to a tweet from Dr. Lipstadt about Senator Ron Johnson, Republican of Wisconsin, who also sits on the Foreign Relations Committee.

In March, Mr. Johnson dismissed the Jan. 6 Capitol riot, saying in a radio interview that he might have felt more threatened had the rioters been “Black Lives Matter and antifa protesters” instead of Trump supporters who “love this country, that truly respect law enforcement.”

Within days, Dr. Lipstadt tweeted a link to an article about Mr. Johnson’s comments and added, “This is white supremacy/nationalism. Pure and simple.”

Republicans are said to be mulling asking Dr. Lipstadt to publicly apologize to Mr. Johnson before allowing her nomination to proceed.

Dr. Lipstadt, 74, is the Dorot professor of modern Jewish history and Holocaust studies at Emory University, and founding director of Emory’s Institute for Jewish Studies. Presidents of both parties have recognized her scholarship and nominated her for leadership roles at the United States Holocaust Memorial Museum.

Dr. Lipstadt has written six books on antisemitism, the Holocaust and Holocaust denial. In 1993, the English writer David Irving sued her and her publisher, Penguin Books, for libel in Britain, after she described him in one of her books as one of the world’s most dangerous Holocaust deniers.

In 2000, Mr. Irving lost the case, in a verdict that was a sweeping condemnation of him and Holocaust denialism. Dr. Lipstadt documented the 10-week trial in her book “History on Trial,” which became the basis of a 2016 film, “Denial.”

Dr. Lipstadt has a long history of using Twitter and other public forums to criticize politicians on the right and left. In 2019, she sharply criticized Representative Ilhan Omar, Democrat of Minnesota, for characterizing pro-Israel Americans as a “political influence in this country that says it is OK for people to push for allegiance to a foreign country.” Such statements are “part of the textbook accusations against Jews,” Dr. Lipstadt told a reporter for Jewish Insider.

Later the same year, after Mr. Trump rejected white supremacy in a statement after shootings in El Paso, and Dayton, Ohio, Dr. Lipstadt told Jewish Insider that his words were insufficient. “While it was good to hear him finally utter those words — white supremacy — lumping this issue with mental health and gun control obscures the fact that white supremacy is amongst the primary, if not the primary, motivating factor of these domestic terrorists,” she said.