Washington - President Donald Trump insisted on Sunday that the US Congress must find a way to pay for his promised border wall, branding Mexico a hotbed of violent crime.And he warned both Mexico and Canada that if they continue to be "difficult" in trade talks, he may have to break off attempts to reform the NAFTA free trade pact.Both positions put Trump on a collision course with both sceptical US lawmakers and neighbouring capitals, but remain popular with his electoral base."With Mexico being one of the highest crime Nations in the world, we must have THE WALL. Mexico will pay for it through reimbursement/other," Trump tweeted."We are in the NAFTA (worst trade deal ever made) renegotiation process with Mexico & Canada. Both being very difficult, may have to terminate?"Trump's positions on the wall and trade are well-known, and both were centrepieces of his campaign for the White House, but they may now run afoul of political reality.US lawmakers return from their summer recess on September 5 needing to come up with a deal to fund the government and raise the federal debt ceiling.The Republican majority wants to cut taxes and pass a costly infrastructure bill, but Trump has threatened to trigger a government shutdown unless it funds his wall.During his campaign, Trump repeatedly insisted that the multi-billion-dollar cost of the wall would be borne by Mexico, an idea scornfully dismissed by Mexican officials.Now - while this remains a centrepiece of his populist speeches - he accepts US funds will pay for construction, and Mexico will be forced to re-pay in some way later on.With Mexico being one of the highest crime Nations in the world, we must have THE WALL. Mexico will pay for it through reimbursement/other.— Donald J. Trump (@realDonaldTrump) August 27, 2017 But Democrats oppose including the wall in the spending bill, and Republican leaders will struggle to pass it, while also opposing a government shutdown.'Work with the Mexicans'Trump's homeland security adviser Tom Bossert attempted to explain the contradiction in an interview with ABC News."What we'll do here is go through the mechanism of getting the original, initial money that we need for that capital project from Congress through an appropriations process and we'll responsibly construct the barrier on the border," he said."As we work with the Mexicans in other policies and trade policies and such, we'll determine ways for us to make that right."At Trump's instigation, the United States, Mexico and Canada began renegotiating the 23-year-old trade deal earlier this month.Trump has repeatedly called the agreement a "disaster" that exported US jobs to Mexico, while Mexico has dismissed his rhetoric as a negotiating tactic playing to his US base.Mexico - which sells 80% of its exports to the US - is not interested in sweeping changes, and Canada is wary.Later on Sunday, Mexico City reiterated it will not be paying for Trump's dream wall."As it has said consistently, our government will not pay for a wall in any way, shape or form; this is not a Mexican negotiating strategy. It's an issue of sovereignty and national dignity," a foreign ministry statement said.