WASHINGTON (AFP) - US Republicans are bracing for their toughest re-election battles in years this November, whipsawed by President Donald Trump's low approval ratings, his propensity to throw the party off message, and exceptional Democratic enthusiasm, experts say.

And Republicans warn that the challenge will grow even tougher should the American leader follow through on his latest economic bluster: threats to impose US$150 billion (S$197 billion) worth of tariffs aimed at China.

Under pressure to retain control of Congress in upcoming mid-term elections, several Republicans cringe at prospects of a protracted trade war between the world's two largest economies slowing US growth and wiping out gains from Mr Trump's tax cut law.

Mr Trump's tariffs could be seen as fulfilling a campaign pledge to protect American industries hammered by globalisation. But a trade standoff could drown out his party's message that tax cuts are delivering prosperity, further imperilling Republicans seeking re-election.

"A trade war that diminishes those gains would be a grievous mistake, and increase the headwinds GOP candidates face in November to gale force," Mr Michael Steel, a former senior aide to Republican House speaker John Boehner and now managing director of consultancy Hamilton Place Strategies, told Agence France-Presse.

The international spat has pummelled global markets, with the Dow Jones Industrial Average tumbling 2.3 per cent last Friday (Apr 6).

Some Republican lawmakers have bluntly opposed their president's trade tiff, expressing fear that Chinese retaliatory duties would hurt several agriculture-dependent states that backed Mr Trump in 2016.

Senator Ben Sasse of Nebraska branded the tariff threats "nuts", while Iowa's Senator Joni Ernst pleaded for the president to reconsider.

'DISCOURAGING REPUBLICANS'

Just how serious an impact could all this have on an election still seven months away is unclear. But analysts are already warning of an economic hit.

A trade war with prohibitively high tariffs on a wide range of products would cut 3.5 per cent from US gross domestic product, according to a recent analysis by the Federal Reserve Bank of Dallas.

The latest ominous sign that Republicans are up against a Democratic wave: respected political forecasters The Cook Political Report last Friday shifted 13 House races towards Democrats.

The party of liberals like Senator Elizabeth Warren and moderates like Montana Governor Steve Bullock is enjoying a "wide voter enthusiasm gap" over Republicans, and young voters - Mr Trump's weakest age segment - are expressing increased interest in casting ballots, wrote Cook political analyst David Wasserman.

The number of House Republican retirements is also outpacing Democrats by two to one, and several of those red seats risk flipping to the opposition.

All 435 House seats are up for grabs in November. Democrats need to gain 24 seats to retake the chamber's majority.

The map is more favourable for Republicans in the Senate. About a third of the chamber's 100 seats are being contested, including 10 held by Democrats from states Mr Trump carried in 2016.

But a lousy political environment and Mr Trump's poor public standing - the latest Gallup poll puts his approval rating at 39 per cent - could forecast Republican disaster.

"The Democrats actually have an excellent chance of flipping the House and, if karma is on their side, even taking the Senate," Iowa State University professor Steffen Schmidt, who has studied US elections for decades, told AFP.

"The consensus is that Trump's persona, the White House turmoil, and the...bad behaviour issues (surrounding some Cabinet members) are discouraging Republicans."

Complicating Mr Trump's woes are the deepening Russia investigation, Mr Trump's alleged affair with porn star Stormy Daniels, and poorer-than-expected jobs numbers for March.

One signal of Republicans difficulties surfaced this week in Tennessee.

Mr Trump carried the southern state by 26 percentage points in 2016, but a recent MTSU poll showed Republican congressman Marsha Blackburn trailing Democratic ex-governor Phil Bredesen by 10 points in their US Senate race.

Senate Majority Leader Mitch McConnell acknowledged the challenging election year ahead.

"We know the wind is going to be in our face," he told Kentucky Today. "We don't know whether it's going to be a Category 3, 4 or 5."

The political hurricane has already walloped Trump country. Democrats picked up a Senate seat in deep-red Alabama in December, won a House race in conservative southwestern Pennsylvania last month, and have made gains in several state legislatures.

Democrats have shattered fundraising records this year in places like Texas, which features a marquee Senate matchup between incumbent Republican Ted Cruz and Democratic challenger Beto O'Rourke.

As their uphill battles loom, several Republicans are bristling at the White House upheaval. But can they be critical of trade policy now without publicly lashing the president?

"If you're a Republican...you're damned if you do, and you're worse if you don't," Mr Doug Heye, a Republican strategist with years of Capitol Hill experience, said last Friday on CNN.

"Do you stand up to Trump and say, 'this is awful and this is not what we want', or do you wait?"