WASHINGTON (Reuters) - Congressional Democrats and President Donald Trump are locked in a constitutional showdown over their powers to investigate him, exchanging threats that present risks for both sides as they head into the 2020 election.

In a clash over the balance of power between the government’s legislative and executive branches, the Trump administration is stonewalling congressional investigators and asserting that it is within its rights to do so.

On Capitol Hill, House Judiciary Committee Chairman Jerrold Nadler, one of several senior Democrats leading probes of Trump, his presidency and his businesses, issued a dire warning:

“The challenge we face is that the president of the United States wants desperately to prevent Congress, a co-equal branch of government, from providing any check whatsoever on even his most reckless decisions,” Nadler said in a hearing on Thursday.

“The very system of government in the United States, the system of limited power, the system of not having a president as a dictator is very much at stake.”

His remarks came after Attorney General William Barr, a Trump appointee, refused to attend the same hearing before Nadler’s committee, which is examining Special Counsel Robert Mueller’s investigation of Russian interference in the 2016 election and Trump’s efforts to stifle the probe.

In an unprecedented approach, Trump in recent days has filed lawsuits meant to block congressional subpoenas that were sent to two banks and an accounting firm that have worked with his businesses, which he did not divest when he took office. The subpoenas seek access to past financial records for Trump.

A businessman-turned-politician, Trump also still refuses to disclose any of his annual tax returns, rejecting decades of practice by recent presidents.

Standing by their president, Republicans in Congress dismissed as hollow Nadler’s rhetoric about Trump’s defiance and played down Barr’s refusal to attend the House hearing.

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The Republicans complained that Nadler wanted committee staff lawyers to be able to question Barr, a departure from the standard hearing format where lawmakers do the questioning.

They stressed Barr’s readiness to defend his handling of the Mueller report before a Republican-controlled Senate panel on the day before he skipped the House hearing.

On Nadler’s comments, Republican Representative Tom Cole said, “It’s over the top. The attorney general showed up before the Senate committee and took every question.”

POLITICAL RISKS

The partisan shouting match in Washington is intensifying as a platoon of Democratic presidential hopefuls hit the campaign trail, with Trump lobbing Twitter insults at the front-runners.

Both sides run risks in ramping up their confrontation. The Democrats could turn off voters if they push too hard to investigate, and perhaps ultimately try to impeach Trump, allowing him to play the victim, a role he excels in.

Former Vice President Joe Biden, the leading Democrat in opinion polls, said this week that Trump’s stonewalling left no alternative but impeachment, which other Democrats have urged.

A Reuters/Ipsos opinion poll showed the public split evenly over impeachment, with 40 percent in favor and 42 percent against it.

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On the other hand, Trump’s behavior may already be worrying Americans. A recent Reuters/Ipsos poll showed Trump with a 37 percent approval rating after the Mueller report’s release, his lowest of the year. [L1N2210EG]

Any further erosion will likely be muted by the economy, which is churning along in its 10th year of expansion. But if economic growth were to falter, the stand-off in Washington could become a bigger issue ahead of the November 2020 election.

MUELLER’S FINDINGS

Mueller’s 448-page report, almost two years in the making, unearthed numerous links between Russians and Trump’s campaign, but concluded there was not enough evidence to establish that the campaign engaged in a criminal conspiracy with Moscow.

It described attempts by Trump to obstruct Mueller’s probe, but stopped short of declaring that Trump had committed a crime.

House Democrats are treating the report as a guide book for more investigations. Shortly after its release in redacted form on April 18, Nadler subpoenaed an unredacted version, as well as the underlying evidence that informed it.

Barr’s Justice Department has refused to comply and Nadler is weighing a contempt citation against Barr over the matter.

In response to Nadler’s and other inquiries, Trump has dug in. In a letter obtained by Reuters, the White House argued that Trump is within his rights to order his advisers not to testify before Congress, even though he allowed them to cooperate with the Mueller investigation.

Meanwhile, House Speaker Nancy Pelosi, a Democrat, accused Barr of lying to lawmakers about his interactions with Mueller. “That’s a crime,” she said.

A Justice Department spokeswoman called Pelosi’s allegation “reckless, irresponsible and false.”

Representative Doug Collins, the top Republican on Nadler’s panel, said Democrats are resorting to hyperbole because the Mueller report did not land a knock-out legal blow on Trump.

“If you don’t have the facts and you don’t have the law, the old joke is that you stand on the table and yell. Well, he’s just standing on the table and yelling now,” Collins said, referring to Nadler.