Tumbling crude and natural gas prices have weakened the energy industry’s influence in Washington as cutbacks in the oilfield have spread to the nation’s capital. Casualties include the army of lobbyists battling new regulations and the rosters of trade groups trying to elect friendly candidates in November. The Independent Petroleum Association of America has shed about 100 members in the past year, as low prices force oil and gas companies to merge, declare bankruptcy and scrutinize every transaction, including as much as $60,000 they send the trade group in annual dues. At the National Stripper Well Association, officials started a monthly payment plan to keep some members on their rolls. But they’ve still lost about 10 percent of their lineup. The National Ocean Industries Association slashed 10 percent off its 2016 dues — the first time it’s given an across-the-board reduction — to try to keep its roster full. […]