Even before the Senate vote, Moscow signaled strong opposition to Montenegro’s efforts. An alleged coup attempt last year that some Montenegrin lawmakers blamed on Moscow was seen as a possible effort by Moscow to undermine the NATO push.
U.S. support for the Montenegro's bid has been mostly strong, though some observers have speculated that President Donald Trump's conciliatory rhetoric toward Russia might result in weaker support for NATO expansion.
However, ahead of the vote, Secretary of State Rex Tillerson wrote Senate leaders, saying approval should come ahead of a key NATO summit scheduled for May.
That would include ex-Soviet republics who have faced Russia’s military aggression: either outright invasion, in the case of Georgia, or a separatist insurgency backed by Moscow, in the case of Ukraine.
"If we were to lose this, it would set back many of the other countries and peoples, particularly in Eastern Europe, who are looking forward to and have their eyes set on the West,” Scaparrotti said.
Moscow appeared to send a signal to both Serbia and others that it remained fully engaged in influencing Balkans politics by hosting Serbian Prime Minister Aleksandar Vucic, to Moscow -- a day before the vote and a week before Belgrade holds a presidential election.
Vucic, who is the favorite in the April 2 vote, has said he wants to bring Serbia closer to the EU but also improve ties with Moscow.