When the Biden administration relaxed some travel restrictions on Americans visiting Cuba in May of last year, Senator Robert Menendez was having none of it.

“I am dismayed,” Mr. Menendez, a New Jersey Democrat, said in a statement. Anyone who believed the measure might help bring democracy to Cuba was “simply in a state of denial,” he fumed.

A day later, Mr. Menendez erupted again, this time over reports that the Biden administration was easing oil sanctions against Venezuela’s authoritarian government — “a strategy destined to fail,” he declared.

For Biden officials, the friendly fire from a fellow Democrat was exasperating if not exactly surprising. Before stepping aside as chairman of the Senate Foreign Relations Committee after his indictment on federal corruption charges last week, Mr. Menendez routinely opposed and even criticized President Biden — and the previous Democrat in the White House, Barack Obama — on foreign policy issues.

From Latin America to the Middle East, Mr. Menendez has long been among the most hawkish Democrats on Capitol Hill, and never afraid to oppose or criticize members of his own party on issues he holds dear. His replacement as chairman of the Foreign Relations Committee, Senator Ben Cardin, Democrat of Maryland, has been vague about his plans but is closer personally to Mr. Biden and likely to be more accommodating of his agenda.

Flexibility has not been Mr. Menendez’s calling card. When Mr. Obama made negotiating a nuclear deal with Iran one of his top second-term foreign policy goals, Mr. Menendez pressed for new sanctions on Tehran that some Obama officials saw as intended to spoil the talks. Once the nuclear deal was completed, in 2015, Mr. Menendez vocally criticized and voted against it. And when Mr. Biden sought in 2021 and last year to return the United States to the agreement after President Donald J. Trump’s withdrawal, Mr. Menendez argued that Mr. Biden was making a dangerous mistake.

Most recently, Mr. Menendez has complicated Mr. Biden’s plans to win Sweden’s admission into the North Atlantic Treaty Organization in what would be a strategic blow to President Vladimir V. Putin of Russia.

Mr. Menendez, who has maintained his innocence, said he would continue to speak out on a range of issues even though he has temporarily stepped down as his committee’s chairman.

“Unless Congress is going to be a rubber stamp for the domestic and foreign policy of any administration,” he said, “it is the constitutional right of Congress to act as a counterweight.”

Benjamin J. Rhodes, a former deputy national security adviser in the Obama White House, said that Mr. Menendez was “a pain on a bunch of issues,” though none more that Mr. Obama’s efforts to restore relations with Cuba. Mr. Menendez, the son of Cuban immigrants, has long advocated a hard U.S. line toward socialist dictators in the region.

“He has used the chairmanship of that committee as a venue for intimidation and retribution to raise the cost of doing anything he doesn’t like,” said Mr. Rhodes, pointing to the control Mr. Menendez has had over whether and when presidential nominees for diplomatic posts would receive hearings in his committee.

Mr. Rhodes and other Democrats are unhappy that Mr. Biden has maintained heavy sanctions placed on Venezuela and Cuba during the Trump administration.

(Mr. Menendez was unable to block Mr. Obama’s Cuba diplomacy or the Iran nuclear deal because he had temporarily relinquished his committee chairmanship during a prior federal corruption investigation. He was acquitted and returned to the position.)

In the near term, Mr. Menendez’s troubles could ease Sweden’s bid to join NATO. Mr. Biden supports the move and all but two members — Turkey and Hungary — have approved it. President Recep Tayyip Erdogan of Turkey complains that Sweden is too welcoming to Kurdish nationalists whom his government considers terrorists.

But Mr. Erdogan says he will green-light Sweden’s NATO membership if the United States agrees to sell his country new F-16 fighter jets along with upgrade kits for existing ones in Turkey’s air force. The issue is set to come up before the Turkish parliament when it reconvenes next month.

Mr. Menendez has long opposed the F-16 sale, citing Mr. Erdogan’s “violent” rule at home and “absolutely awful” policies abroad, including his aggressive use of American-bought warplanes in Cyprus and against U.S.-allied Kurdish forces in Syria. That position put Mr. Menendez out of step with some other Democratic members of his committee, who believe the F-16 deal should be approved if Mr. Erdogan agrees to Sweden’s NATO membership.

Mr. Erdogan cheered the senator’s demotion this week, telling reporters that “Menendez being out of the picture is an advantage” for Turkey.

Mr. Cardin may take a less stringent position. Speaking to reporters on Thursday, he called the issue “complicated.”

Even if Mr. Cardin adopts a softer line, obstacles remain: Mr. Menendez’s counterpart in the House, Representative Gregory W. Meeks, Democrat of New York, and the ranking member of the Foreign Affairs Committee, said Tuesday that he remained skeptical about the F-16 deal.

Mr. Menendez’s loss of control over his committee also creates possible new openings for the Biden administration in the realm of sanctions policy.

On Iran, Mr. Menendez has teamed up with Republicans to codify sanctions against Iran’s ballistic missile and drone development program before U.N. penalties for those programs expire next month. That effort could limit the Biden administration’s ability to negotiate with Tehran on its nuclear program and other matters at a time when the White House has sought to de-escalate tensions with the country.

While Mr. Cardin has expressed interest in seeing those sanctions extended, he has not signed on to that legislation.

Mr. Cardin, who was filling in for Mr. Menendez as the top Democrat on the Foreign Relations Committee when the 2015 nuclear deal was approved, also voted against that agreement. But he was less critical of the deal than Mr. Menendez was, calling his own decision “a close call.”

On Cuba, Mr. Cardin asked reporters this week to “give me a little bit of time,” but noted that he holds a “pro-engagement” view toward Havana — hardly Mr. Menendez’s philosophy.

If Mr. Menendez’s legal woes are reason for some celebration in Turkey, they are cause for concern in Armenia, which is losing its most powerful congressional advocate at a moment of crisis after Azerbaijan this month captured the long-disputed Armenian enclave of Nagorno-Karabakh.

Mr. Menendez has strong political and personal ties to Armenia: New Jersey has a large Armenian population and he is married to an ethnic Armenian, Nadine Arslanian. (Ms. Arslanian is also his accused co-conspirator). He has long supported officially recognizing that Turkey committed genocide against Armenians in the early 1900s and other matters dear to the small Christian nation in the Caucasus.

As Azerbaijan blockaded the Armenian residents of Nagorno-Karabakh in recent months, Mr. Menendez pressed for a much firmer U.S. response — hauling a senior State Department official in for a grilling before his committee and accusing the United States of pandering to Azerbaijan’s president, Ilham Aliyev.

He also introduced a bill to send humanitarian and military aid to Armenia while imposing sanctions on Azerbaijan in response to what the measure calls a campaign of “ethnic cleansing.”

But the legislation’s prognosis likely depends on it having a dogged champion willing to prioritize it on the committee’s schedule. Mr. Cardin is not listed among the co-sponsors of the bill.

On Thursday, he said that while he was opposed to extending a waiver that would allow the Biden administration to provide otherwise-prohibited military assistance to Azerbaijan, he would defer making decisions on other matters regarding Nagorno-Karabakh until he could talk to other members and the Biden administration.