NEW DELHI—Two U.S. senators have urged President Joe Biden to waive sanctions against India over its purchase of Russia’s S-400 air defense system, saying such a punitive measure would endanger growing cooperation. India signed a $5.5 billion deal with Russia in 2018 for five of the surface-to-air missile systems for defense against longtime adversary Pakistan, and China, with which it is locked in a standoff on their disputed border. The proposed transfer has caused friction with the United States, which passed a law in 2017 under which any country engaged with Russia’s defense and intelligence sectors could face sanctions. Republican Senator John Cornyn and Democratic Senator Mark Warner wrote a letter to Biden on Tuesday calling for a waiver on the grounds of national security and broader cooperation. “We believe there is a national security imperative to waiving sanctions,” the senators said in their letter. They said they were concerned …