WASHINGTON, Dec 4 (Reuters) - President-elect Donald Trump's pick to lead the U.S. Department of Defense, Pete Hegseth, on Wednesday said he is not backing down even as his nomination to lead the Pentagon faces trouble in Congress over allegations about his personal and professional life. "I’m doing this for the war fighters, not the warmongers. The Left is afraid of disrupters and change agents. They are afraid of @realDonaldTrump — and me. So they smear w/ fake, anonymous sources & BS stories. They don’t want truth. Our warriors never back down, & neither will I," the former Fox News...