Ukrainian President Volodymyr Zelensky told U.S. senators visiting Kyiv this week that waiting to impose sanctions on Russia until after an invasion is of no use to Ukraine, according to four sources familiar with the discussions.

Why it matters: The Senate is currently working on a major sanctions package to deter Russia from attacking Ukraine. Democrats and Republicans are united in their support for Ukraine, but divided over whether it would be more effective to sanction Russia now to signal resolve, or hold up the threat of future sanctions to demonstrate the high costs of an invasion.

State of play: The Biden administration and its European allies have warned they will impose severe consequences if Russia invades, but have yet to lay out specific measures that all sides can agree on.

• Sen. Bob Menendez (D-N.J.), chairman of the Senate Foreign Relations Committee, has introduced what he calls the "mother of all sanctions" bill, which would go into effect if President Biden determines that Russia has escalated its hostilities toward Ukraine.
• His Republican counterpart, Sen. Jim Risch (R-Idaho), has introduced legislation that would immediately increase military support for Ukraine and impose sanctions on the Nord Stream 2 pipeline, which the Ukrainians view as an existential threat to their security.
• Between the lines: The Democratic bill would ultimately give Biden discretion on when to impose sanctions, which he already has the authority to do; the Republican bill would force the president's hand now.

Behind the scenes: Zelensky expressed gratitude to the seven senators who made the trip for their bipartisan support, but argued that real deterrence would mean imposing immediate costs to signal to Vladimir Putin that the U.S. is not to be trifled with.

• "The president was very clear: Nord Stream must be sanctioned now. And any prospective sanction of Nord Stream or other sanctions are not helpful, whether it's a policy or a law," a source close to Zelensky told Axios.
• Sanctions that are conditional on further escalation "are at best neutral, at worst actually emboldening Putin," the source added, contending that, as neighbors, the Ukrainians have a better understanding of "the Russian mentality."
• "Putin does not understand these kind of vague threats. 'If' is not in his vocabulary. There's 'yes' and there's 'no.'"

The other side: A senior Democratic aide hit back at Zelensky's message, telling Axios: "If anybody should understand the perils of interfering in the legislative process of another country, it should be President Zelensky."

The big picture: Another senior Democratic aide told Axios that the overall tenor of the meeting with Zelensky was positive and warm, despite the disagreement on when sanctions should be triggered.

• "The focus of the policy of the U.S. administration is about building the toughest sanctions package that can be levied in coordination with the European allies, because whether we like it or not, the Europeans actually have the stronger hand here with respect to sanctions that can really hurt Russia," the aide said.
• The source close to Zelensky also stressed that the meeting was positive and the Ukrainian government appreciates the bipartisan support.

What to watch: Sen. Chris Murphy (D-Conn.) told reporters on a briefing call Monday that he was returning from Kyiv "pretty confident that the Senate can come together on a set of prospective deterrent sanctions against Russia."

• Republicans on the trip have echoed that sentiment.
• But if an agreement is possible, they'll have to move fast: The Senate will recess on Friday until Feb. 21, at which point a Russian invasion could already be underway.