"The only thing Putin respects is strength ... At this point the Russians are openly laughing at the president," Cruz, R-Texas, told the Washington Examiner.
Cruz added that Putin's aggressiveness was "a direct consequence" of the absence of American leadership in the world.
"The weakness and incoherent foreign policy of the Obama administration -- from President Obama, under Secretary of State Hillary Clinton and under Secretary [John] Kerry -- has undermined our allies and has strengthened our enemies and put Putin in particular in a far stronger position," Cruz declared.
Cruz acknowledged that he was glad Obama was "reluctantly" and "slowly" moving toward acting on Ukraine, but added that he should be doing more.
He suggested that Obama sign a free trade agreement with Ukraine, develop natural gas infrastructure to export resources to Ukraine, and restore a missile defense program in Eastern Europe.
"I think Putin's aggressiveness and act of war against Ukraine is a direct consequence of the absence of U.S. leadership," Cruz said. "For five years, America has receded from leadership in the world, and into that vacuum has stepped in nations like Russia, Iran, and China — and has made the world a much more dangerous place."