RANCHO MIRAGE, Calif. — President Barack Obama declared Tuesday that Republicans have no constitutional grounds to refuse to vote on a Supreme Court nominee, and he challenged his political foes in the Senate to rise above the "venom and rancor" that has paralyzed judicial nominations. As Obama cast the dispute over filling the seat of the late Justice Antonin Scalia as a test of whether the Senate could function, there were early signs that Republican resistance could be eroding. Senate Judiciary Chairman Charles Grassley suggested he might be open to considering Obama's yet-to-be named nominee, an indication his party may be sensitive to Democrats' escalating charges of unchecked obstructionism.