WASHINGTON — Chinese Vice Premier Liu He goes to Washington for trade talks on Thursday and Friday, setting up a last-ditch bid for progress toward a deal that would avoid a sharp increase in tariffs on Chinese goods ordered by U.S. President Donald Trump. U.S. officials have accused China of reneging in the past week on substantial commitments made during months of negotiations to end their trade war, prompting Trump to set a new deadline to raise tariffs. …Differences over the enforcement mechanism for a trade deal and a timeline for tariff removal have been sticking points. …Many free trade deals have built-in dispute settlement mechanisms…Canada, for example, has brought challenges to U.S. anti-subsidy duties on softwood lumber before panels set up under NAFTA’s Chapter 19. …President Trump frequently threatened to quit NAFTA during negotiations last year… any move to quit NAFTA is likely to draw a court challenge.