Most Republicans in the US Senate have taken the path of least resistance - staying silent - as they grapple with President Donald Trump's swirling impeachment firestorm.

In a way they hold the key to Trump's political fortune: Should the president get impeached by the Democratic-led House of Representatives, he would then face a trial in the Senate, the Republican-held chamber he views as his firewall.

While Democrats unite in outrage over the US leader's latest brazen move, a public call for China to investigate his political rival Joe Biden and his son, outright condemnation by Republicans was rare.

Republican senators' silence signaled to those in their home states that while many in Trump's party may be uncomfortable with his willingness to flout political norms and seek foreign help in the 2020 election, they were offering a form of quiet defense of the president.

Constituent frustration appeared to catch some Republicans off guard, including Senator Joni Ernst, who seeks re-election in 2020.

"You still stand there silent and your silence is supporting him," one person told Ernst at a town hall Thursday in her state of Iowa.

"Where is the line?" asked voter Amy Haskins. "When are you guys going to say, 'Enough?'"

"I can't speak for him," parried Ernst.

"But you can speak for yourself," Haskins interrupted.

Pressed for a reaction to Trump's bald suggestion Thursday that China, and earlier Ukraine, investigate his potential 2020 White House challenger, Ernst reverted to the unfounded White House talking point that the Bidens were involved in corruption in those countries.

"Corruption, no matter where it happens, must be fought everywhere," she said.

Senator Marco Rubio downplayed the controversy, suggesting Trump might have even made the China comments in jest.

"I don't know if that's a real request or him just needling the press," Rubio said.

While neutral reactions or indirect support of Trump have been common this week, full-throated backing is more elusive.

Loyal ally Senator Lindsey Graham hesitated to defend Trump's call for China to investigate the Bidens, but he sought to explain the request.

"It's the president pushing back," Graham told The Washington Post. "He feels like everyone is coming after him all the time and he hasn't done anything wrong."

The president issued a barely veiled warning Friday to those who hold the majority in the Senate, where he would be tried if impeached by the House.

"We have a great relationship in the Senate," he said.

"I have a 95 percent approval rating in the Republican Party," Trump added.

While he has yet to speak to many senators on the issue, Trump said, "I believe the senators look at this as a hoax."

Few Senate Republicans are openly castigating him, and election politics is likely a significant reason why.

Twenty-three Republican Senate seats are in play in 2020, compared to just 12 for Democrats, and those in the GOP standing for re-election are loath to cross a president who enjoys an extremely loyal base.

But some voices are rising up to challenge him.

Trump's "brazen and unprecedented appeal to China and to Ukraine to investigate Joe Biden is wrong and appalling," tweeted prominent Senator Mitt Romney, the Republican presidential nominee in 2012 and currently the party's most open critic of Trump.

Fellow Republican Ben Sasse, at greater risk because he is up for re-election next year, issued some of the harshest language yet attacking Trump's China comments.

"Hold up: Americans don't look to Chinese commies for the truth," Sasse told the Omaha World-Herald.

"If the Biden kid broke laws by selling his name to Beijing, that's a matter for American courts, not communist tyrants running torture camps."

The fiery comments made headlines, but they may not move the needle.

For Trump to be ousted by the Senate, a unified Democratic caucus will need to at least 20 Republicans to defect to their side, a target that currently appears unlikely to be met.

Copyright Agence France-Presse, 2019