A top Republican senator has placed a hold on the nomination of Nicholas Burns, President Joe Biden’s pick to be the U.S. ambassador to China, over what he called the diplomat’s “failure to understand” the threat posed by the Chinese Communist Party.

Sen. Marco Rubio of Florida, ranking member on the Senate Intelligence Committee, hit the pause button on the confirmation process for Burns this week following Burns’s October appearance before the Senate Foreign Relations Committee and one day after Biden’s three-hour discussion with Chinese leader Xi Jinping. Burns, an ambassador during the Clinton administration and a State Department official during George W. Bush’s presidency, is thought to have wide support in the Senate, but Rubio said the Biden nominee is not the right man for the job.