Nebraska Sen. Deb Fischer on Monday endorsed Marco Rubio, hailing the Florida senator as the strongest commander-in-chief in the Republican field.Fischer, who sits on the Senate’s Armed Services Committee, praised Rubio for his ability to inspire others and his depth of knowledge on national security.“I know what the challenges are out there that we face, and no one knows these threats better, no one knows the challenges that we have in the world today — the dangerous world we face — than Marco Rubio,” she told Fox News’ “America’s Newsroom” on Monday.