President Biden — who has made renewed international engagement a hallmark of his foreign policy ethos — is headed to a pair of global summits in Europe this week with just a handful of his ambassadors in place, as most of his picks to represent the United States abroad remain mired in messy domestic politics.

To date, only four of Biden’s choices to be a U.S. ambassador to a foreign government have been approved by the Senate — three of them just on Tuesday. That means Biden is lagging considerably behind his immediate predecessor, Donald Trump, who at this point in his presidency had 22 such U.S. ambassadors confirmed, 17 of them by voice vote, according to data compiled by Senate Democratic leadership aides.

The delays stem from threats by some Republican senators, led by Ted Cruz (Tex.), who has been angling for a fight with the Biden administration over matters of national security. That is prolonging the usually routine process of getting ambassadors formally installed, while several high-profile posts are also vacant because the White House has yet to put forward nominees for them.

The vacancies are coming into sharper view this week as the president embarks on the second overseas trip of his term, first for the Group of 20 summit in Rome and then to Glasgow, Scotland, for the United Nations climate summit known as COP26.

As President Biden heads to COP26, he faces tough political challenges. White House reporter Annie Linskey and London Bureau Chief William Booth explain. (Zach Purser Brown/The Washington Post)

Among the other 19 members of the G-20, 15 of them do not have a U.S. ambassador in place (Indonesia and Russia have U.S. ambassadors who were held over from the Trump administration). Biden has yet to nominate his own pick for Italy, which this year is hosting the annual gathering of leaders from the world’s largest economies, nor for the European Union, United Kingdom, South Korea, Saudi Arabia, Brazil and Australia.

The four ambassadors to foreign governments — Mexico, Turkey, New Zealand and Austria — who have been confirmed are either former senators or the widows of former senators, whom Cruz said he would not block as a courtesy. Cindy McCain, the widow of former senator John McCain (R-Ariz.), was also approved Tuesday as the U.S. representative to a United Nations food agency, which gives her the rank as ambassador. And the Senate approved Linda Thomas-Greenfield as United Nations ambassador as part of the Cabinet confirmations earlier this year.

But as time passes without the Senate processing ambassadors for elsewhere in the globe, allies of the administration are increasingly sounding the alarm about the diplomatic ramifications. Meanwhile, Democratic senators’ usual irritation at Cruz has reached new levels.

“This risks being hyperbolic, but it’s like negotiating with a terrorist,” Sen. Chris Murphy (D-Conn.) said of Cruz, a member of the Senate Foreign Relations Committee with past and potentially future presidential ambitions. “He is not the secretary of state. The people of this country did not elect him or his party to represent us abroad. And what he’s asking for is to control American foreign policy.”

Senate Foreign Relations Committee Chairman Robert Menendez (D-N.J.) added, “At the end of the day, it’s not my job to make his presidential ambitions happen.”

In some countries, high-ranking government officials will not meet with anyone short of a formal U.S. ambassador, shunning the chargés d’affaires who have taken over in the interim.

U.S. ambassadors can often attract attention from the foreign press and public that chargés cannot — and ultimately, they exert more influence while promoting the United States’ agenda and explaining decisions made back home to foreign leaders. The bipartisan 9/11 Commission has suggested that delays in confirming national security and foreign policy nominees can damage the United States’ ability to respond to attacks.

“Public diplomacy is neutered when you don’t have an ambassador,” said Murphy, a senior member of the Senate Foreign Relations Committee. “When six months or a year goes by without a U.S. ambassador, they infer that it’s a value judgment being placed on the relationship.”

Cruz’s main goal is to force the administration to slap sanctions on the Russia-backed company behind Nord Stream 2, a Russia-to-Germany gas pipeline that critics say could empower Moscow and give it significant leverage in Europe.

The Biden administration waived sanctions over Nord Stream 2 in May. In an interview, Cruz said he was using his leverage to stop Biden from making what he called a “generational geopolitical mistake.”

“This surrender to Putin won’t just impact today or tomorrow or next year, 10 years from now,” Cruz said. “Thirty, 40, 50 years from now, there will be Russian dictators, earning tens of billions of dollars from Nord Stream 2 and using it to fund military aggression against America and our allies and using it to hold Europe subject to energy blackmail.”

Cruz has spoken with Treasury Secretary Janet L. Yellen and Jake Sullivan, the national security adviser, and has met privately with Deputy Treasury Secretary Wally Adeyemo in a secure facility at the Capitol to discuss the issue. The senator said he has offered to release some of his holds on nominees if the administration takes steps under a 2017 sanctions law to effectively waive the sanctions but trigger a congressional vote to override the decision. But that could put some Democrats in a politically awkward spot.

Senate Democrats are plotting how best to exert pressure on Cruz to stop his blockade of potentially dozens of critical, foreign policy picks. The Texas senator said in an interview last week that he had holds on seven pending envoys, although since then, more than 30 ambassadors and other State Department picks have advanced to the Senate floor, which means Cruz may be holding up roughly three dozen nominees. Cruz’s office declined to provide specific numbers.

The senator had told The Washington Post last week he would not block people such as former senator Jeff Flake (R-Ariz.) to Turkey and former senator Tom Udall (D-N.M.) out of senatorial courtesy. Victoria Reggie Kennedy, also confirmed Tuesday as U.S. ambassador to Austria, is the widow of former senator Ted Kennedy (D-Mass.).

No one senator can indefinitely block a nominee as long as the person has majority support in the Senate. But Cruz’s holds mean that unless he relents, the Senate will be forced to eat up valuable floor time to confirm nominees that usually sail through unanimously.

The White House blasted the senator for his far-reaching action.

“President Biden is quickly working to restore America’s position on the world stage, and he’s moved swiftly to nominate well-qualified ambassadors who have earned Republican and Democratic support,” White House spokesman Chris Meagher said. “But instead of putting politics aside, Sen. Cruz has led an unprecedented effort of obstruction by blocking dozens of President Biden’s nominees and preventing them from advancing America’s national security interests.”

There has been no indication that Biden administration officials are willing to give in to Cruz’s demands. Nor should they, according to Democratic senators.

“You can hold up the universe,” Menendez said. “But you’re also going to pay the price when something happens in the world and you were the one holding that particular nominee.”

Cruz, unsurprisingly, is unbothered by the criticism.

“One of the things it says is that the Biden administration is really bad at getting their work done and moving their nominees forward,” Cruz said, when asked about the ambassador vacancies. “And Senate Democrats have been quite slow in processing those nominees.”

Meanwhile, Senate Republicans haven’t been eager to intervene to get Cruz to back down, but they haven’t, for the most part, bolstered his efforts either. Some GOP senators are quick to note the benefits of having Senate-confirmed ambassadors to represent U.S. interests abroad.

Sen. Rob Portman (R-Ohio), a Foreign Relations Committee member, said he recently met with former interior secretary and Democratic senator Ken Salazar, who was confirmed in August as ambassador to Mexico, to discuss two issues important to the retiring Ohio senator: migration and the proliferation of fentanyl.

“It’s useful to have somebody there who has some stature,” Portman said, adding that Salazar had just met with Mexican President Andrés Manuel López Obrador when the two men spoke.

Cruz also does not plan to hold up the nomination of Nicholas Burns to be U.S. ambassador to China, one of the most critical diplomatic relationships, according to a person familiar with the decision.

But those, for now, are probably going to be the exceptions. As of Wednesday, Biden had nominated 111 people for State Department jobs, but has only had 24 of them confirmed, according to data provided by a White House official. State picks who have been successfully confirmed under this president have had to wait an average of 108 days from start to finish.

That was almost double the amount of time that State Department nominees under Trump waited to be confirmed — 56 days — at this point in his term, which in turn was longer than the average for presidents Barack Obama (43 days) and George W. Bush (27 days).

“This is such a dangerous moment for American relations with the world,” said Brett Bruen, a former Obama State Department official. “If we don’t have the right people out there representing us, I think we are at risk of losing an extraordinary amount of credibility and influence in the world.”

Bruen also pins some blame on the Biden administration, saying it was slow to get the ambassador application process started for career officials and also that Biden so far has tapped a disproportionate number of political ambassadors for posts abroad, compared to career Foreign Service officers.

“The career people I’m talking to say, ‘What about us?’ ” Bruen said.

If Cruz and other like-minded Republicans don’t cease their blockade, Senate Majority Leader Charles E. Schumer (D-N.Y.) has told Democratic senators that they will have to confirm these noncontroversial nominees “the hard way” and work late nights and weekends to process them, according to a person familiar with the majority leader’s thinking.

That moment, the person said, is likely to come soon.

Meanwhile, for dozens of administration nominees in the queue, their lives have been put on hold, with limits on how they can earn a salary and what they can say publicly as they navigate Senate confirmation politics. Their children may not know where they would attend school.

“This is incredibly frustrating. It’s demoralizing,” said one State Department nominee going through the Senate process. The person spoke on the condition of anonymity because of the pending confirmation. “Whatever you think of people and the qualifications, these are people who are willing to serve, and they’re willing to serve in places that are not the most appealing places to be.”