U.S. ambassador’s post to the Vatican remains unfilled after Miguel Diaz, a theologian, left the job to teach at the University of Dayton November. (TONY GENTILE/REUTERS)

Vice President Biden attended Tuesday’s installation of Pope Francis at the Vatican. House Minority Leader Nancy Pelosi did, too, along with other lawmakers.

But there was no U.S. ambassador in the crowd at St. Peter’s Square. That post has been vacant since November, when Miguel Diaz, a theologian, left the job to teach at the University of Dayton.

Some Catholic critics of the Obama administration see that empty chair as a symbol for the lack of engagement between Washington and Vatican City.

One of the candidates for ambassador, Nick Cafardi, former dean of the Duquesne University School of Law and co-chair of Catholics for Obama, disputed that characterization, listing numerous human rights campaigns in which the Vatican works with the State Department, such as battling human trafficking. In addition, while battling poverty is hardly a new focus for the Vatican, some speculate whether Pope Francis’s emphasis on the issue could become the basis for even more cooperation.

“The Holy See typically does not base its diplomatic relations on theology,” Cafardi said, adding that the Vatican “at one point sent an ambassador to Genghis Khan, for heaven’s sake.”

A few U.S. bishops might not object to such a comparison.

Bishop Daniel Jenky of Peoria, Ill., for example, compared President Obama to Hitler and Stalin last year. Cardinal Timothy Dolan of New York, mentioned as a papal contender leading up to the conclave, has said that the White House is “strangling the Catholic Church.”

And Bishop Salvatore Cordileone of Oakland, Calif., has said there is reason to fear “despotism” under Obama.

But relations between the Vatican and the administration are not as difficult as those between the U.S. bishops and the White House, even if Pope Benedict XVI said to U.S. bishops last year that “many of you have pointed out that concerted efforts have been made to deny the right of conscientious objection . . . with regard to cooperation in intrinsically evil practices.”

He was referring to abortion, by virtue of the Affordable Care Act’s provision of insurance coverage for contraception, including the “morning-after” pill, which the U.S. bishops — unlike their German counterparts — consider an abortifacient.

It is often said among observers that the Vatican is to the right of the Republican Party on social policy but to the left of the Democratic Party on economic issues. But the Vatican’s differences with Washington during the George W. Bush administration — notably, over the war in Iraq, which Pope John Paul II saw as unjust — never cooled relations the way the view of some bishops who see Obama as “the abortion president” has.

“They’re strained,” Steve ­Schneck of Catholic University said of the relationship between the White House and the Holy See.

But some of the problems have been exacerbated because the Vatican’s view is colored by some U.S. bishops’ accounts, said ­Schneck, director of the school’s Institute for Policy Research and Catholic Studies.

For example, he said, insurance coverage of contraceptive is “not particularly different from programs in Europe or Latin America — the moral differences from the Catholic perspective are nil — yet the Vatican sees domestic policy primarily through the eyes of American bishops.”

Schneck said the White House is working to repair the relationship. But the fact that the ambassador job has been vacant for several months has not helped.

A onetime candidate for the job, Schneck said he learned from the White House’s personnel office on Friday that he was out of the running.

Cafardi said he had been told there would be a decision on an ambassador by St. Patrick’s Day.

The personnel office said last week that it had no announcement on the position.

Other candidates for the job include Ken Hackett, former president of Catholic Relief Services, and Rep. Daniel Lipinski (D-Ill.), co-chair of the Congressional Pro-Life Caucus.

The White House press office referred questions about the relationship to the State Department; a spokesman there responded with the statement of congratulations that Secretary of State John F. Kerry made after Pope Francis was chosen.

The Vatican could not be reached for comment.

Biden, back from Vatican City and hosting a belated St. Patrick’s Day party at the vice-presidential residence, told reporters Wednesday that he had met the new pope and that “he seems like all he is advertised to be — a genuine, generous, humble man.”