The new government in Kiev has sought military assistance from the United States after Russian forces invaded Ukraine's Crimean peninsula and absorbed it, sending regional tensions soaring.
To date, the Pentagon has said it would only consider non-lethal aid to Kiev.
But hawkish Republican Senators John McCain and Lindsey Graham, citing reports that Russian troops are massing near Ukraine's border, argued that President Vladimir Putin is issuing a threat that must be countered.
Aid should include "small arms, ammunition, and defensive weapons, such as anti-armor and anti-aircraft systems" as well as non-lethal support including protective equipment and intelligence sharing capability.
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"Events in Crimea and the growing Russian threat in Europe's east must be a wake-up call to NATO," which they said should shift assets and capabilities eastward.
But McCain and Graham, never ones to mince words over national security, offered a blunter warning: "Considering President Putin's track record of lies and aggression, it is prudent now to expect and plan for the worst."
McCain and Graham have long-called for the White House to arm Syrian rebels fighting the forces of President Bashar al-Assad, who has the Kremlin's backing in Syria's civil war.
The senators and other critics have argued that Obama's failure to enforce his red line that Assad crossed last year by using chemical weapons has emboldened Putin.