By Nandita Bose, Ted Hesson and Costas Pitas WASHINGTON (Reuters) -President Joe Biden said on Friday that the border deal being negotiated in the U.S. Senate was the “toughest and fairest” set of reforms possible and vowed to “shut down the border” the day he signs the bill. The bipartisan talks have hit a critical point amid mounting Republican opposition. Some Republicans have set a deal on border security as a condition for further Ukraine aid. Earlier in the day, U.S. House Speaker Mike Johnson said the deal is “dead on arrival” in its current form, according to a letter to Republican law…