* Obama, Republicans differ on border security * Immigration suddenly a priority in Washington * US has 11 million illegal immigrants * Obama launches immigration reform in Las Vegas (Adds more details) By Matt Spetalnick LAS VEGAS, Jan 29 (Reuters) - Just over a week into his second term, President Barack Obama took his fight for immigration reform to the West on Tuesday and pushed Congress to quickly find a path to citizenship for 11 million undocumented residents.