WASHINGTON - Republican and Democratic senators plan to introduce legislation that would force US President Donald Trump to obtain Congress' approval before imposing tariffs on national security grounds, a senior senator said on Tuesday. The president decided last month to open a trade investigation into whether auto imports had damaged the US auto industry, which could lead to tariffs of up to 25 percent on "national security grounds". That has worried some Republican lawmakers who strongly back free trade, warning that Trump could trigger a trade war that would destabilize the economy and ultimately hurt American workers. White House spokeswoman Sarah Sanders said Trump was still looking at the possibility of doing individual trade deals with Canada and Mexico in place of NAFTA. "He's looking at the best way to make sure he gets the best deal possible for American workers," Sanders said.