Since Russia invaded his country three years ago, President Volodymyr Zelensky of Ukraine has survived a military assault on his capital, assassination plots, corruption scandals in his government, political infighting and ominous setbacks in his army’s fight against Russia.

He had enough support from Ukrainians to carry him through each time.

Now, with Donald J. Trump installed in the White House, Mr. Zelensky is facing a new challenge: maintaining good relations with the country’s most critical ally and a president who has been disdainful toward him and skeptical of military aid.

Mr. Trump’s arrival comes at a precarious time for Mr. Zelensky domestically. The soaring popularity he experienced early in the war — with an approval rating of about 90 percent — has been slumping badly. The latest polling shows support sinking to nearly 50 percent, and it falls even lower in surveys that gauge his popularity against potential competitors if elections were held in the wake of a cease-fire agreement with Russia.

And a new trouble spot for Mr. Zelensky has emerged: the revival of political opposition in Ukraine, animated by the prospects of a cease-fire and the elections that could soon follow. His opponents are also encouraged by the barrage of criticism Mr. Trump and his aides have aimed at Mr. Zelensky.

Two opponents who ran against Mr. Zelensky in Ukrainian elections in 2019 — former President Petro O. Poroshenko and former Prime Minister Yulia Tymoshenko — have reached out to members of Mr. Trump’s team. Ms. Tymoshenko traveled to Washington to attend some inaugural events on Monday.

Mr. Zelensky did not attend the ceremony. He had said he would travel to Washington only if invited by Mr. Trump.

“He believes in the one-man show, but it doesn’t work,” Oleksiy Goncharenko, a member of Parliament in the opposition European Solidarity party, said of Mr. Zelensky’s role as the face of Ukrainian resistance after the Russian invasion in 2022. More pluralism will help the war effort, he said in an interview: “We are not Russia.”

Mr. Zelensky rallied his people and allied nations through the war with nightly videos and frequent trips abroad. But beyond that, he has cloistered himself in an ever-tightening circle of loyal aides, restricting access to opposition figures and typically ignoring their advice, Mr. Goncharenko said. The more active opposition emerging now will aid the war effort, he added.

To be sure, no vote in Ukraine is scheduled — or even possible, election experts say — while the war rages and the country is under martial law. Russia could disrupt any voting with missile volleys. Millions of Ukrainians, including soldiers in combat, refugees in Europe and people living under occupation, would risk disenfranchisement. So while Ukrainians are fighting for their democracy, they cannot practice it.

Still, opposition figures have not failed to notice how setbacks in the war have whittled away at Mr. Zelensky’s popularity. Under the Constitution, elections must be called after martial law is lifted. Parliament first imposed martial in February 2022, after the full-scale Russian invasion, and extends it with periodic votes.

By one measure, Mr. Zelensky still has the support of a majority of Ukrainians, albeit a slim one: 52 percent still have trust in the president, according to a poll in December by the Kyiv International Institute of Sociology.

But polling focused more narrowly on a hypothetical presidential election shows Mr. Zelensky trailing a former commander in the military, Valery Zaluzhny, who was removed by the president as part of a sweeping overhaul of the military command and is now Ukraine’s ambassador to Britain.

One survey, by the polling agency Leading Legal Initiatives, showed Mr. Zaluzhny winning a hypothetical first round of a two-stage election with 24 percent of the vote. Mr. Zelensky trailed, with 16 percent; and Ms. Tymoshenko, the opposition figure, came in third place with 12 percent. Neither Mr. Zaluzhny nor Ms. Tymoshenko has declared an intention to run.

Dwindling support has implications beyond politics for Mr. Zelensky: It could undermine his role as commander in chief in wartime.

“It is hardly worth explaining further what disasters can happen in the event of delegitimization and collapse in control,” Anton Hrushetskyi, the executive director of the Kyiv International Institute of Sociology, wrote in an analysis of falling approval ratings.

That has not stopped Mr. Trump and members of his entourage from taking swipes at Mr. Zelensky. At a rally in September, for example, Mr. Trump called Mr. Zelensky “the greatest salesman in history” for the billions in military aid he has secured to defend his country.

Speaking to reporters on Monday after the inauguration, however, Mr. Trump offered a positive assessment of Mr. Zelensky’s openness to settlement talks and one of his harshest assessments to date of Russia’s president, Vladimir V. Putin, who he said was “destroying Russia” with the war.

Mr. Zelensky and aides have scrambled to make inroads with Mr. Trump’s team. The Ukrainian president met with Mr. Trump in New York in September. Andriy Yermak, the Ukrainian presidential chief of staff, met in December on Capitol Hill with Vice President-elect JD Vance and the incoming national security adviser, Michael Waltz, who was then a congressman from Florida, according to two people familiar with the meeting.

Mr. Zelensky’s aides have also looked to curry favor behind the scenes. Officials in Kyiv have discussed the possibility of brokering a deal to purchase the Ukrainian language rights to Melania Trump’s book, “Melania,” according to a Ukrainian official familiar with the discussion.

It was unclear if any in the group had reached out to Ms. Trump’s aides or publisher, the official said. He described the discussion as not more than “brainstorming” by Zelensky aides on establishing good relations with Mr. Trump.

On a trip to Washington in December, Mr. Poroshenko spoke with Mr. Waltz, the pick for national security adviser. Mr. Poroshenko touted the exchange in a Facebook post in which he promoted his ties to the Trump team and included photographs of himself with Mr. Waltz.

“I received assurances of the readiness of the new American Administration to demonstrate leadership in the matter of repelling Russian aggression and establishing a just peace in Ukraine,” he wrote, adding that Mr. Trump’s team “remembers well our cooperation with him during my presidency.”

A person who was present said Mr. Poroshenko had overstated the significance of the interaction with Mr. Waltz, which occurred in a hallway at a function the two were attending and was not a formal meeting. Mr. Poroshenko has denied pursing any political goals during the war.

Oleksandr Merezhko, the chairman of the foreign policy committee in Ukraine’s Parliament and a member of Mr. Zelensky’s political party, said it was standard diplomacy for Mr. Trump’s team to engage with an opposition party.

“Ukraine is a democratic society,” he said. “It’s great to meet with the opposition.”

But he claimed that the opposition’s outreach was “mostly about self-promotion and political PR.” Mr. Merezhko has tried a different approach to focus Mr. Trump on the war: Last fall, he nominated Mr. Trump for a Nobel Peace Prize.

Mr. Merezhko said he did not believe that Mr. Trump held any deep-seated animosity toward Mr. Zelensky. The “greatest salesman” comment, he said, could be read as laudatory.

“In the world of Trump, this is a compliment,” he said.