The US Senate Intelligence Committee’s decision to subpoena Donald Trump Jr has ignited an internal Republican firefight over the fate of the committee’s Russia probe, as the panel’s GOP chairman showed no signs of backing down despite fierce criticism from many of his colleagues that it was time to move on.

The sudden infighting threatened to undermine support for the Senate’s Russia investigation, which is the sole bipartisan probe in Congress into Russian interference in the 2016 election and has been widely praised as operating with little public drama.

Much of the backlash against the decision by Chairman Richard Burr of North Carolina to subpoena Donald Trump’s eldest son came from GOP senators who are up for re-election next year and from those closely aligned with the president.

The outrage was partially fuelled by Mr Trump Jr and his own allies.

“This would not go forward without Republican complicity,” Senator Rand Paul, an ally of Mr Trump’s, told reporters on Capitol Hill. “So I think it’s a mistake for Republicans to keep putting the Trump family through this, and I really think they ought to drop it.”

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The abrupt disclosure this week of the Trump Jr subpoena – issued at least a week ago, according to people familiar with the situation – came shortly after Senate Majority Leader Mitch McConnell insisted that he considered as closed all matters investigated by special counsel Robert Mueller.

Yet, until Thursday, the Senate Intelligence Committee has faced little public pressure to wrap up its investigation, which began in January 2017.

And some GOP senators on the powerful but secretive committee made a point to stress publicly that their probe was separate from Mr Mueller’s, despite attempts by others to link the two.

Marco Rubio, a member of the intelligence panel, said the intense criticism of Mr Burr was in part a misunderstanding of the focus of the committee’s investigation, which Senator Rubio said is being inaccurately conflated with the special counsel probe.

“Mueller is a criminal justice investigation,” Mr Rubio said. “Ours is an intelligence investigation about the Russia threat and about the way our agencies performed.”

Some other Senate Republicans, on and off the Intelligence Committee, also defended Mr Burr.

“I’m not going to comment on what actions are being taken,” said Susan Collins, who sits on the committee. “But I support his leadership and think that he has done a good job.”

Mitt Romney, who is not on the committee, said: “I have confidence in Chairman Burr, and if he’s requesting testimony, I presume he has a reason for that.”

Still, the conservative resistance against Mr Burr’s subpoena continued to intensify Thursday.

The president's son is said to be “exasperated” at the subpoena, according to a person who has discussed it with him.

And the president repeatedly stressed on Thursday that he was “surprised” by the demand, considering that his eldest son had testified for “hours and hours”.

“I sure wish Sen Richard Burr was as interested in Biden’s cushy deals w/Ukraine & China while VP than he is in the harassment of @DonaldJTrumpJr over a CLOSED witch hunt,” tweeted former Arkansas governor Mike Huckabee, the father of White House press secretary Sarah Sanders.

In tweets, Charlie Kirk, the founder of the pro-Trump organisation Turning Point USA and a friend of Mr Trump Jr’s, singled out Republican senators who are running for re-election in 2020 and taunted them to fall into line with the Trump family against Mr Burr’s subpoena.

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“Conservatives are watching closely how @SenThomTillis responds to his North Carolina colleague @SenatorBurr’s senseless targeting of @DonaldJTrumpJr,” Mr Kirk said in one of this tweets. “Primaries will not be kind to Republicans who stand silent as government power is...abused to harass the President’s family.”

Mr Tillis, who is already facing a primary challenge from the right, said on Thursday morning that he disagreed with the committee’s decision to subpoena Trump Jr.

“We have a 400-plus page report that was determined after about $30m (£23m) of money spent, dozens of investigators, hundreds of subpoenas and inquisitions that there was no underlying crime and no obstruction,” Mr Tillis said.

“I personally believe Democrats are just trying to keep this thing alive, and it’s their latest launch point to do it.”

But when a reporter noted that it was the committee led by Mr Burr, a Republican, that issued the subpoena, Mr Tillis responded: “I think you’d have to speak to Senator Burr. I stand by my comment.”

Senator John Cornyn, a member of Mr Burr’s committee who is also up for re-election in 2020, said on Thursday morning that he was not aware in advance that a subpoena had been issued for Trump Jr but said: “At some point, this is not about finding facts.”

“This smacks of politics,” Mr Cornyn said, according to CBS News. “And I think we have an important job to do to try to keep the Intelligence Committee out of politics.”

Asked to clarify, Mr Cornyn said he was not accusing Burr of playing politics.

A Cornyn spokesman said he was “saying at some point the congressional investigations smack of politics, not specifically this decision”.

Senate Judiciary Committee Chairman Lindsey Graham stressed that he would not second-guess Mr Burr but said in his view, “Mueller’s the final word for me”.

“If I were Don Jr’s lawyer, I’d be reluctant to jump back into this circus,” Mr Graham added. “It’s just crazy. Mueller spent two years and $25m, and to me, he’s the final word.”

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The GOP tensions could escalate further if the president’s eldest son defies the subpoena and Mr Burr triggers a showdown by insisting on enforcing the summons – leaving it in the hands of Mr McConnell to decide whether to hold Mr Trump Jr in contempt of Congress.

Asked whether Mr Burr cleared the subpoena with Mr McConnell in advance, a spokesman for the majority leader said Mr McConnell has not directed Mr Burr or the committee on anything during the probe. The spokesman, David Popp, also declined to say whether Mr McConnell would put a contempt resolution on the Senate floor if Mr Trump Jr defied the demand, calling that scenario hypothetical.

“I think the good news is Chairman Burr has already indicated the committee will find no collusion,” Mr McConnell said in a Fox News interview on Thursday night.

“I think this is going to have a happy ending...I understand the president’s frustration here but I think this is just a blip.”

Mr Burr repeatedly declined to speak with reporters on Thursday.

But at a closed-door lunch of GOP senators, he explained the reasoning behind the subpoena, according to people familiar with the discussion – including the timeline of the negotiations with Mr Trump Jr over his repeat testimony.

Also during the Thursday lunch, Mr McConnell told his colleagues he had faith in Burr’s leadership, encouraging them to stop their public attacks on his decision to issue a subpoena, according to people familiar with the conversations.

In an interview this week conducted before the revelation of the Trump Jr subpoena, Mr Burr – who was elected to a third term in 2016 and has said he will not run again – dismissed any notion that politics is influencing his investigation.

“My responsibility is to the institution and the equities of the committee, and I’m judged by the product that we produce and how we carry out the investigation,” Mr Burr said, adding that he could not let politics “influence what people think of the final report”.

The panel issued the subpoena after weeks of negotiations with Mr Trump Jr for a second interview that aides to both parties say he had to have known was always part of the deal, since the committee always planned to bring back key witnesses for a second session in which senators would be included.

Committee staffers have been conducting most of the interviews in the intelligence panel’s probe.

Jared Kushner, Mr Trump’s son-in-law, appeared for a second closed-door session in late March.

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But the panel eventually lost its patience with Mr Trump Jr, issuing the subpoena more than a week ago.

The summons became public only a day after Mr McConnell delivered a speech declaring “case closed” in regard to congressional investigations after the completion of the Mueller report.

However, Mr McConnell also acknowledged an exception for the Intelligence Committee’s probe, which he said should continue.

“Hopefully Don Jr and his lawyers will believe it’s important,” said Roy Blunt, who also sits on the committee.

“I think his brother-in-law came in, in a similar situation, to be sure that every question was clearly understood and clearly answered, and I think he should, too.”

Mark Warner, the committee’s vice chairman, also expressed confidence on Thursday that Burr would hold the line against fresh attacks from within his own party.

“Since the beginning of this investigation, both of us have faced pressure at times – him to shut down the investigation, me to draw conclusions before we’re finished,” Mr Warner said. “We’re going to do our job.”

Part of that job, Mr Warner said, was to ensure that the elections were “protected in 2020” – the same cycle in which most of Burr’s critics will be facing their next political test.