WASHINGTON » When Sen. Robert Menendez of New Jersey said last week he would give President Barack Obama two months before defying a veto threat and voting for new sanctions on Iran, he made it clear that the delay was not out of loyalty to his fellow Democrat in the Oval Office.

"I don’t get calls from the White House," Menendez said.

It was a frank acknowledgment of the rifts that exist between Obama and Menendez, the top Democrat on the Senate Foreign Relations Committee. The divisions have burst into public view in recent weeks as Menendez, a second-term senator, has taken on Obama over Cuba and Iran.

Obama’s advisers say they speak with Menendez regularly, and the senator himself described his relationship with the White House as "excellent." But deep policy and political divisions remain between Obama and the senator, one of the Democrats best positioned to defend the administration’s foreign policy in Congress.

During a Foreign Relations subcommittee hearing Tuesday, Menendez sharply questioned State Department officials about Obama’s move to open diplomatic relations with Cuba, calling it "a bad deal" that "compromised bedrock principles for virtually no concessions." In December, Menendez, the son of Cuban immigrants who has made opposition to the Castro regime a centerpiece of his political life, said Obama’s decision had "vindicated the brutal behavior of the Cuban government."

He has also battled with Obama over a bill to impose new sanctions on Iran, which the president argues would undermine talks to prevent Tehran from acquiring a nuclear weapon. Menendez is the bill’s co-sponsor.

And during Obama’s State of the Union address last month, Menendez sat grim-faced as other Democrats cheered the president’s promise to veto any effort to roll back his domestic agenda. Days later, Menendez told Obama administration officials they sounded as if they were peddling "talking points that come straight out of Tehran" in arguing against his sanctions bill.

All the while, Menendez has agitated for a more aggressive response to Russia’s aggression in Ukraine, calling for more sanctions on Moscow and military assistance to Kiev, among other measures.

Katie Beirne Fallon, Obama’s chief liaison to Congress, said, "There are a few high-profile things where we don’t agree, but on 98 percent of the issues, we work very closely and well." Menendez, she added, was a "fierce defender" of the president’s domestic agenda.

The public disputes on foreign policy issues pose difficulties for Obama that are both substantial and symbolic, said former Sen. Evan Bayh, D-Ind.

"It makes the president’s challenge of communicating his position to the American people more difficult when prominent members in his own camp are taking a different point of view," Bayh said.

Republican senators and aides say it has been politically valuable to have Menendez’s public support, often in opposition to the White House.

"Any time a senator from the other party will say something like that, it is helpful," said Sen. Lindsey Graham, R-S.C. Sen. Bob Corker, R-Tenn., and chairman of the Foreign Relations Committee, called Menendez "a great partner."

Tommy Vietor, a former National Security Council aide in the Obama administration, said that could be a problem, given that Sen. John McCain, R-Ariz., has vowed to use his power as chairman of the Armed Services Committee to challenge Obama on virtually every foreign policy issue.

"To have the top Democrat at Senate Foreign Relations doing similar things is a tough political situation to be in, that’s for sure," Vietor said. "The good news is that I think year-six Barack Obama cares as little about politics as he possibly can, especially when it comes to foreign policy decision-making."

There has never been much personal or political rapport between Obama, 53, and Menendez, 61, who even allies say is most comfortable when in a fight.

Menendez endorsed Hillary Rodham Clinton, a longtime ally, in the 2008 Democratic presidential race over Obama, who had campaigned for him in New Jersey.

In 2010, Menendez and the president clashed after Martha Coakley, the Democratic candidate in a special election for Sen. Edward M. Kennedy’s seat in Massachusetts, lost a lopsided election to Scott Brown. A White House official was quoted anonymously in blaming Menendez, then the chairman of the party’s Senate campaign arm, for the loss.

Menendez has also battled with the Justice Department over a public corruption investigation he said was instigated by Cuban spies to discredit him.

At the same time, aides to Menendez say he has been a reliable partner to the president, moving scores of his nominees through the Foreign Relations Committee and engineering bipartisan agreements on tricky international issues, like the authorization of the use of force in Syria in 2013.

"He is a center of gravity, the only guy who can bridge the left and the right and get something forward," Adam Sharon, Menendez’s communications director, said of the senator. "It’s not confrontational or adversarial or being a thorn in the president’s side, but him being a leader, bridging the divides so that something can get done."

But Menendez has taken umbrage at recent treatment by the president.

The senator, who was invited by Obama to travel with him on Air Force One during a visit to Lakehurst, N.J., two days before the shift in Cuba policy was announced, has since suggested that he is angry to have been left out of the talks that preceded it.

"To be notified when it’s going to happen is not consultation," he said last week.

Menendez also said he took "personal offense" at the president’s suggestion during a closed-door exchange last month that supporters of the Iran sanctions bill were motivated by politics. Some of the people there interpreted the comment as a thinly veiled reference to pressure from pro-Israel groups that back a hard line against Iran. (Menendez has received $341,170 over the past seven years from such groups, more than any Democrat in the Senate, according to Maplight, a nonpartisan research group.)

His pledge last week, in a letter also signed by nine other Democratic senators, to delay an Iran sanctions vote until late March gave Obama the breathing room he had been seeking on the nuclear talks with Tehran.

But it also came with a threat: If no deal is struck by then, Menendez will side with Republicans to approve the sanctions, bringing like-minded Democrats with him. "It’s our intention to move forward at that time," he said.

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