(CNN) Senate Republicans are standing by President Donald Trump in his escalating trade war with China -- but they aren't prepared to back his other protectionist impulses.

Trump faced a Saturday deadline to decide whether to move ahead with threatened tariffs of up to 25% on cars coming from Europe, a move many fear would alienate key US allies and devastate global supply chains, as the President is trying to resolve brewing crises in Iran and North Korea

But Trump plans to delay his decision on auto tariffs by six months, according to a person familiar with the situation, in order to pursue negotiations with the European Union and Japan.

The delay will give Congress six months to take action to limit Trump's ability to move unilaterally on the tariffs, in the event that talks with the EU -- which Trump's top trade negotiator has already said are at "a complete stalemate" -- fall apart.

Republicans have long been more deferential to Trump on China -- a country they agree should be held accountable for intellectual property theft and other unfair trading practices. But they don't feel that logic applies to Trump's other trade moves , including steel and aluminum tariffs imposed last year on national security grounds, under Section 232 of the Cold War-era Trade Expansion Act.

"We would look back and say, 'OK, this pain was worth it because we ended up with a really good agreement and much better Chinese behavior,'" Pennsylvania Republican Sen. Pat Toomey told reporters on Tuesday. "That's what we can still hope for with respect to China. But with respect to the 232 tariffs that we've seen on steel and aluminum against our allies and nearest neighbors and closest friends, I'm afraid we can't make that case at all."

Republicans view duties on auto imports as a doomsday scenario that could cripple the economy if paired with the ongoing tensions with China. If Trump follows through on his threats, it could spur congressional action to limit his power on the issue.

"I would be very surprised and extremely disappointed if we decided that Volkswagens are a threat to America's national security and we imposed a tax on American consumers in that category," Toomey said.

Republicans have pushed back on the White House's previous use of Section 232, though they have not advanced legislation on the matter.

GOP leaders shot down a bill to require congressional approval for the national security tariffs last year, spearheaded by retired Tennessee Republican Sen. Bob Corker, because members did not want to take a difficult vote before the 2018 midterm elections. Toomey has introduced a revamped version of the bill.

Ohio Republican Sen. Rob Portman has meanwhile advocated a less direct approach, which would not require congressional approval for the tariffs. Senate Finance Committee Chairman Chuck Grassley says he plans to introduce a compromise bill to blend the two, but negotiations have moved slowly.

"We're not there yet," Portman told CNN on Tuesday.

Portman, a former US trade representative, also explained why he was more willing to get behind the White House on combating Chinese trade practices.

"If you look at the 301 case that was filed, it is very specific on what China is doing on subsidization, on tech transfer and so on," Portman, who was US trade representative during the Bush administration, told CNN of the distinction between China and Trump's other national security tariffs. "It's a different approach, different animal."

On China, they've expressed willingness to go along with Trump's roller coaster ride in hopes that he can reach the comprehensive deal he's promised.

"Ultimately, nobody wins a trade war unless there is an agreement at the end, after which tariffs go away," Senate Majority Leader Mitch McConnell told reporters on Tuesday.

Texas Sen. John Cornyn, when asked how patient senators are willing to be given the pain for farmers and other interests, told CNN: "I'm not sure this is entirely up to us."

Other senators said they welcomed the crackdown on China despite the pain it has created for farmers and other US businesses -- as well as higher prices for consumers.

"The President is right to hold China's feet the fire on this," Wyoming Sen. John Barrasso told CNN. "They wouldn't be negotiating at all if it weren't for what the President has done. ... The President has his own timeline. I support what he is doing.

Democrats who oppose Trump's trade decisions say the GOP's failure to legislate on the issue is indicative of its members' timidity towards Trump.

"I'm done listening to Republicans complain about this," Sen. Chris Murphy, a Connecticut Democrat, told reporters of tariffs on Tuesday. "They have the power to do something about it."

He added: "There's a range of options, and Republicans are pretending as if they're not senators. They want to complain about the tariffs, but they don't actually want to do anything about it."