About 5,000 American troops have rotated through Estonia since April 2014 — after Russia’s seizure of Crimea — and Mr. McCain said he would support a permanent United States troop presence, not just a rotation.
“I think the presence of United States troops here in Estonia is a signal that we believe in what Ronald Reagan believed, and that is peace through strength,” he said, calling it “the best way to prevent Russian misbehavior.”
The Russian takeover of Crimea, along with its efforts to destabilize eastern Ukraine and its continued threats to Georgia, reinforced the need for the United States to help keep NATO strong, Mr. McCain said. “Our relationship is more important than perhaps it’s been in a long time,” he said.
Spokesmen for the senators and Mr. Trump did not respond to questions about whether they had informed the president-elect about their trip.
Mr. McCain also referred to the consensus among American intelligence agencies that Russia tried to interfere in the presidential election, a contention that Mr. Trump has rejected.
“There’s no doubt that the Russians were hacking,” Mr. McCain said, though he added that he saw “no evidence” that “Russian cyberattacks and leaking of information had any tangible effect on the outcome.”
Toomas Hendrik Ilves, who ended a 10-year term as Estonia’s president in October, said in an interview that the Senate delegation was “more than a business-as-usual visit.”
But Mr. Ilves also played down the idea that the Baltic countries were anxious. “Estonians are not nervous — at best, apprehensive in the face of ambiguous messages,” he said. “Recall that Donald Trump also backed off on his initial statement and, after all, he was talking about those who do not pay enough for defense. We do.”
Mr. Trump said during the presidential campaign that NATO members must pay their share if they want the benefits of a collective defense. Estonia is one of the few NATO members that meet or exceed the benchmark of spending on its military, at least 2 percent of gross domestic spending. (The others are the United States, Britain, Poland and Greece.)
Audronius Azubalis, a Lithuanian lawmaker and former minister of foreign affairs, said in a telephone interview that the senators’ visit “sends a strong message to the Baltic states and those under direct threat — Georgia and Ukraine — that the allies will stand with us” and that the United States would not bow to Russian aggression.
Lithuanians have been alarmed by Mr. Trump’s comments in support of President Vladimir V. Putin of Russia, Mr. Azubalis said, but are now waiting to see if his attitude toward Mr. Putin shifts once he takes office.
“That was the campaign,” Mr. Azubalis said, “now let’s give the president-elect time to show his real intentions.” Should he continue to embrace Mr. Putin, Mr. Azubalis said, “that could be a threat for the world.”
Ojars Eriks Kalnins, the chairman of the foreign affairs committee of the Latvian Parliament and a former Latvian ambassador to the United States, noted in a phone interview that members of both parties in Congress had tried to reassure the Baltic states for months that American policy toward European security would not change.
“Mr. Trump’s positions seem to change fairly regularly, and so it’s possible he’s moderated a lot of earlier positions,” Mr. Kalnins said. “Our hope is that his administration will come around to backing the ongoing policy toward NATO and the Baltics.”
He added: “I don’t think there is a plausible scenario for Russia invading the Baltics. To be honest, I’m more concerned about the fate of Ukraine.”
But some analysts contended that Mr. Trump had already caused considerable damage to European security.
“The worst thing that Trump has done is question the American commitment to NATO deterrence,” said Edward Lucas, a senior vice president at the Center for European Policy Analysis. “Without America, NATO doesn’t work. NATO is basically a force multiplier for America in Europe.”
Jorge Benitez, a scholar at the Atlantic Council in Washington, said that the Baltic countries were looking for “clarity and commitment,” and that the senators were limited in how much assurance they could provide.
“While congressional support for NATO can be reassuring to our allies and may influence some of the new president’s view on our alliance commitments,” he said, “the bottom line is that the decisive power to defend or abandon NATO belongs to President Trump.”