By Steve Holland NASHUA, New Hampshire (Reuters) – A U.S. congressman looking to thwart President Joe Biden, a fellow Democrat, from a second term assailed Biden as weak and unelectable on Saturday as he tried to take advantage of Biden’s absence from New Hampshire’s first-in-the-nation primary. Biden is skipping the New Hampshire primary after the Republican-run state refused his demand to give up its first in nation primary spot to South Carolina. But Biden’s supporters are mounting a write-in campaign to avoid an embarrassing loss in the state on primary election day on Tuesday. Congressman…