WASHINGTON (Reuters) - The National Rifle Association endorsed Republican Donald Trump for U.S. president on Friday and he responded by promising to serve the country’s top gun lobby group well.  Trump, his party’s presumptive nominee for the Nov. 8  presidential election, praised the NRA and took a swipe at Democratic front-runner Hillary Clinton, saying she wants to abolish the U.S. Constitution’s Second Amendment, which guarantees the right to bear arms. The billionaire businessman has opposed gun-free zones and promised to protect Americans’ rights to self-defense. “I will not let you down,” Trump told the gathering in Louisville, Kentucky. The NRA is the country’s leading gun-rights advocacy group.  