As Crimea held a referendum all but certain to endorse secession from Ukraine, a bipartisan group of eight senators returned from a weekend trip to the region and called for quick congressional action on a package of aid for Ukraine and sanctions against Russia.

The trip "helped foster some bipartisanship that I think will help advance the sanctions," said Sen. John Hoeven (R-N.D.), one of the members of the delegation, which was led by Sen. John McCain (R-Ariz.).

Upon their return, four of the senators appeared on Sunday morning talk shows to increase the sense of urgency surrounding the bill, which passed the Senate Foreign Relations Committee last week but has gotten tangled in a separate set of issues involving changes to the International Monetary Fund that some Republicans oppose.

In an interview, Hoeven said that if the parties cannot work out their differences over the IMF provisions, which would shift $63 billion from its crisis fund to its general accounts, they should set that issue aside and move ahead of the other parts of the measure.

Meanwhile, McCain said on CNN's "State of the Union" that "economic sanctions are a very important step. Identify these kleptocrats and - look, Russia is a gas station masquerading as a country. Its kleptocracy, its corruption, it's a nation that's really only dependent upon oil and gas for their economy. And so economic sanctions are important."

The senators warned that the consequences of inaction would be dire.

Sen. Richard J. Durbin (D-Ill.), appearing on NBC's "Meet the Press," called Russian President Vladimir Putin's recent actions in Ukraine "the single most serious act of aggression since the Cold War. He ended up the final ceremony at the Sochi Olympics, which are network-covered, trying to make it a charm offensive for the world that this is a modern Russian nation. And within hours he’s invading one of his neighbors, sending the same troops that were protecting the athletes at Sochi into the Crimea. Now, are we going to stand by and say this is acceptable conduct? Because this isn't the end of his ambition.​"

At the same time, some of them acknowledged that the penalties may not be enough.

"It’s going to be difficult," said Sen. Jeff Flake (R-Ariz.), who appeared with Durbin. "Let’s face it, Russia has always had its design on Crimea, it considers Crimea a part of Russia. And so that’s going to be difficult. But all you can do is increase the cost significantly, and -- and hope that they don't move further into Ukraine. . . . We've got to move in hard now with sanctions, regardless of whether they move tomorrow or the next week or hold back. We're going to move forward with sanctions; not just us, but our European allies as well."