WASHINGTON — Counting down to a high-stakes Crimean referendum, President Obama declared Wednesday that the U.S. would “completely reject” a vote opening the door for the strategic Ukrainian peninsula to join Russia if the election goes ahead on Sunday. Adding pressure on Russia, the Senate advanced a package of potentially tough economic sanctions against Moscow.

Obama made a point of welcoming Ukraine’s new leader to the White House, declaring as they sat side-by-side that he hoped there would be a “rethinking” by Russian President Vladimir Putin of the referendum. If the vote does occur, Obama said, the international community “will be forced to apply a cost to Russia’s violation of international law.”

Secretary of State John Kerry also was talking tough, telling Congress, “It can get ugly fast if the wrong choices are made, and it can get ugly in multiple directions.” Kerry will meet with Russian Foreign Minister Sergey Lavrov on Friday in London in a last-ditch effort to halt the referendum.

Amid the maneuvering, Obama met in the Oval Office with new Prime Minister Arseniy Yatsenyuk, praising him and the Ukrainian people as the two sat for TV coverage. The meeting was aimed at showcasing the United States’ commitment to Ukraine, the former Soviet republic at the center of rising tensions between East and West.

“There’s another path available and we hope President Putin is willing to seize that path,” Obama said. “But if he does not, I’m very confident that the international community will stand firmly behind the Ukrainian government.”

As Obama and Yatsenyuk met, a Senate committee moved forward on legislation with potential punishments for Russia. The measure, which next would go to the full Senate, would authorize $1 billion in loan guarantees to Ukraine’s new government and allow the Obama administration to impose economic penalties on Russian officials responsible for the intervention in Crimea.