In a letter to Obama on Thursday, the senators argued that, since Russian President Vladimir PutinVladimir Vladimirovich PutinRussia says coronavirus vaccine will be ready for doctors in two weeks Democrats ramp up warnings on Russian election meddling Fauci: 'I seriously doubt' Russia's coronavirus vaccine is safe and effective MORE is providing “heavy weapons” to separatists in Ukraine, the least the United States could do is send “defensive” weapons to the Ukrainian military.
“We should supply the Ukrainian military with appropriate defensive weapons such as anti-tank weapons to help them reclaim their territory,” McCain and Donnelly wrote. “We encourage you to provide appropriate intelligence to Ukraine to ensure it has the necessary support to counter effectively a rebel force wholly dependent on Russian backing.”
Russia has already taken control of Crimea and now is sending a convoy to eastern Ukraine despite, international criticism. 
The United States has issued sanctions against Russia and has urged its European allies to do the same as part of an effort to stop Putin. The administration has also sent Ukraine nonlethal military assistance, such as night-vision goggles, body armor and first aid kits. But McCain and Donnelly said the United States should do more.
“With Russia increasing its military presence on the Ukrainian border, by some estimates to as many as 40,000 troops and heavy weapons, there is growing concern that Russia could soon invade,” the letter stated. “Our enhanced support will send a strong message to President Putin that efforts to expand or cement Russia’s influence through foreign aggression will not succeed.”