Richard Wolf

USA TODAY

WASHINGTON -- The White House and leaders in Congress vowed Sunday to impose tough economic sanctions on Russia in hopes of stopping further aggression into Ukraine, following what they decried as a bogus referendum in Crimea.

President Obama spoke with Russian President Vladimir Putin on Sunday afternoon, telling him the Crimean referendum would never be recognized by the United States and the international community.

Obama urged diplomacy, citing Ukraine's upcoming elections and constitutional reform as steps toward de-escalation, but said a diplomatic resolution isn't possible with Russian military in Ukrainian territory.

"We reject the 'referendum' that took place today in the Crimean region of Ukraine," White House press secretary Jay Carney said. "This referendum is contrary to Ukraine's constitution, and the international community will not recognize the results of a poll administered under threats of violence and intimidation."

Carney said the administration is urging other nations to join in economic sanctions, which President Obama has threatened to impose. Key lawmakers in both political parties also urged passage of an aid package for Ukraine, possibly including military assistance.

"This referendum is in violation of international law. The United States is not going to recognize the results," said White House counselor Dan Pfeiffer on NBC's Meet the Press. "We are working with our partners around the world, the Europeans in particular, to marshal forces against the Russians."

The question will be whether European allies have the resolve to follow through on sanctions. Many of them are dependent on Russia for energy supplies.

The presidents of the European Council and European Commission issued a joint statement Sunday condemning the referendum but stopping short of promising concrete action. They said foreign ministers would "evaluate the situation" at a meeting in Brussels Monday "and decide on additional measures."

British Foreign Minister William Hague said at that meeting, "measures must be adopted that send a strong signal to Russia that this challenge to the sovereignty and territorial integrity of Ukraine will bring economic and political consequences."

The Senate Foreign Relations Committee approved a package of economic aid for Ukraine and sanctions against Russia last Wednesday, but Congress adjourned for a recess without completing action. Lawmakers aren't due to return until March 24.

ECONOMIC SANCTIONS, THEN MILITARY AID?

There was no shortage of recommendations from Republicans as well as Democrats in Congress: freezing overseas assets of Russian leaders and business entities, providing military assistance to Ukraine, resuming a dormant U.S. missile defense system in Eastern Europe, and creating a path toward NATO membership for Georgia and Moldavia were mentioned.

"The United States of America, first of all, has to have a fundamental reassessment of our relationship with Vladimir Putin," Sen. John McCain, R-Ariz., said on CNN's State of the Union. "No more reset buttons, no more tell Vladimir 'I'll be more flexible.' Treat him for what he is."

Several lawmakers predicted that if Russia annexes Crimea following Sunday's vote, it might next look to southern and eastern Ukraine. "This isn't the end of his ambition," said Sen. Richard Durbin, D-Ill., on NBC.

Senate Foreign Relations Committee Chairman Robert Menendez, D-N.J., said the repercussions might not stop with Russia.

"The Chinese are watching and saying, 'Let's see what the West does, because ultimately we've got territory in the South China Sea that we believe is ours," Menendez said on Fox News Sunday.

His House counterpart, Foreign Affairs Committee Chairman Ed Royce, R-Calif., denounced the Crimean vote as "a throwback to the Soviet era. No vote occurring under military occupation deserves to be treated as legitimate."

Sen. Chris Murphy, D-Conn., who just returned from a trip to Ukraine, called the referendum "a threat to the territorial integrity of Europe. Who knows who's next?"

In response, Murphy said, the U.S. and its allies must vote as early as Monday to "take a chunk of flesh" out of the Russian economy.

"There is no doubt that if you cut off Russian gas to Europe, it will hurt," he said. "There's no doubt that if you freeze Russian assets in places like Germany and Great Britain, it will hurt them."

FROM SOVIET SPHERE TO NATO'S BORDER

Ukraine, the largest and most industrialized of the former Soviet republics that declared their independence in 1991, was a top target for NATO membership during the administration of President George W. Bush. But those efforts were blocked by Russia, as well as France and Germany, which doubted Ukraine's readiness to join the military alliance.

During a trip to Kiev in 2008, Bush belittled the sight of protesters waving Communist flags.

"Just because there was a bunch of, you know, Soviet-era flags in the street yesterday ... you shouldn't read anything into that," Bush quipped.

That 1991 independence vote was just one of six such Ukrainian declarations during the past century, five of which failed.

"Very often, Ukraine looked like a diversified country, a parted country," former President Viktor Yushchenko once said.

Ukraine's recently toppled government did lean back toward Russia and Putin, who called for Sunday's referendum on short notice in hopes of putting Crimea back into the former Soviet sphere.

"God knows what Vladimir Putin will do next," McCain said. "He believes that Ukraine is a vital part of his vision of the Russian empire, and we need to understand that and act accordingly."