mexico city leaders mexico canada thursday pledge work toward renegotiation north american free trade agreement nafta even talk take place unite state turn sour due hard-line u.s. demand speak mexico city fourth round talk rehash nafta hold near washington canadian prime minister justin trudeau mexican president enrique pena nieto say commit win-win-win deal talk unite state hit obstacles tensions increase dramatically thursday due u.s. demand include add so-called sunset clause nafta would force negotiations trillion pact every five years two source direct knowledge talk arlington virginia describe atmosphere horrible highly charge walk away table base proposals put forward trudeau say news conference response question whether clause poison pill talk ask whether bilateral deal possible talk fail pena nieto say mexico would keep work canada unite state reach deal beneficial three countries suggest could ways move forward think canada mexico share nafta agreement good mechanism one good mechanism boost development region pena nieto say u.s. president donald trump say nafta originally sign 1994 disaster unite state frequently threaten scrap unless major change make besides sunset clause unite state also want modify dispute settlement mechanisms boost amount u.s. content autos must contain qualify tax-free status business leaders three countries say u.s. proposals could derail talk early thursday mexican finance minister jose antonio meade say mexico analyze tariff import substitution plan case nafta scrap possibility identify tariff measure possibility identify market providers market turn meade tell senators capital work work perfect analysis identify industries company could help contingency reach satisfactory negotiation meade give detail tariff analyze 2011 mexico successfully use target tariff u.s. goods pork cheese win dispute truck meade comment tensions talk drive peso nearly five-month low 18.92 dollar fall percent session currency shed close percent since oct. concern nafta talk could founder canada mexico vow stick nafta talk mexico work plan