June 29 (Reuters) - South Dakota's Republican Governor Kristi Noem said on Tuesday up to 50 National Guard troops from the state were being sent to Texas to help secure the U.S.-Mexico border, in a deployment paid for by a private donation. The move comes amid a hail of criticism from high-profile Republicans of Democratic President Joe Biden for rolling back immigration restrictions introduced by his predecessor Donald Trump, with the number of migrants arriving at the border reaching the highest monthly levels in two decades. "The Biden Administration has failed in the most basic duty of the federal government: keeping the American people safe," said Noem, who was a vocal supporter of Trump's hardline immigration policies.