Trump faces allegations of encouraging Ukraine to take action against a leading Democratic challenger.

Pelosi warns that "a whole new stage of investigation" may be ahead.

Trump and Giuliani counter with furious attacks on Biden and his son.

The Russia investigation? Settled, more or less.

Cue Ukraine.

Once again, Donald Trump faces explosive allegations of encouraging foreign meddling to help him win a presidential campaign. Once again, the reports are fueling Democratic calls for his impeachment. And once again, the president is responding with a furious counter-attack on the Democratic rival involved.

On Sunday, Trump confirmed that he had discussed Democratic presidential frontrunner Joe Biden during a congratulatory phone call on July 25 with the new Ukrainian president, Volodymyr Zelensky. The conversation was largely about corruption, he told reporters while en route to a Texas rally, "all of the corruption taking place, largely the fact that we don't want our people like Vice President Biden and his son creating to the corruption already in the Ukraine."

A still-unnamed administration official in an intelligence agency was so alarmed by Trump’s interactions with a foreign leader that he or she filed a whistleblower complaint.

The Washington Post has reported the complaints concerns Trump’s communications with Ukraine, though the substance of it is shrouded in secrecy. The administration has enraged congressional Democrats by refusing to turn that complaint over to oversight committees in Congress, as the law requires.

That has stoked new demands for the president's impeachment, including some from lawmakers who had resisted endorsing the idea in the wake of special counsel Robert Mueller's investigation of Russian interference in the 2016 election. His report and his low-key testimony before Congress failed to galvanize public momentum for removing Trump from office, as some Democrats had hoped.

In a remarkable bit of timing, Mueller's decisive congressional testimony was on July 24. Trump's phone call with the Ukrainian president took place the next day.

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"If in particular, after having sought foreign assistance and welcomed foreign assistance in the last presidential campaign as a candidate, he is now doing the same thing again but now using the power of the presidency, then he may force us to go down this road," House Intelligence Chairman Adam Schiff, D-Calif., said on CNN's State of the Union. In the past, Schiff has declined to call for impeachment until there was a critical mass of public support behind it. "I have spoken with a number of my colleagues over the last week, and this seems different in kind and we may very well have crossed the Rubicon here."

House Speaker Nancy Pelosi, who has cautioned against impeachment unless it has bipartisan support, struck a warning note in a "Dear Colleague" letter released Sunday that called on the acting director of National Intelligence, Joseph Maguire, to turn over the whistleblowers' complaint when he is scheduled to appear before Congress Thursday.

If the administration refuses, she said, "they will be entering a grave new chapter of lawlessness which will take us into a whole new stage of investigation."

It's hard to know whether the Ukrainian furor will sway public opinion in a way the Russian allegations did not, or if it might chip away at the president's nearly solid wall of support among congressional Republicans. The Russian controversy didn't do that, either.

"Look, it is not appropriate for any candidate for federal office, certainly including a sitting president, to ask for assistance from a foreign country," Republican Sen. Pat Toomey of Pennsylvania said on NBC's Meet the Press in one of the few public comments Sunday from a congressional Republican. "That's not appropriate. But I don't know that that's what happened here."

Utah Sen. Mitt Romney made a similar point on Twitter Sunday afternoon. "If the President asked or pressured Ukraine's president to investigate his political rival, either directly or through his personal attorney, it would be troubling in the extreme," said Romney, the GOP's presidential nominee in 2012. "Critical for the facts to come out."

Trump insists he did nothing wrong, and he and his personal attorney, Rudy Giuliani, are trying to shift the focus from the president's behavior to Biden's past. They accuse the former vice president of improperly acting to boost the business interests in China and Ukraine of his son, Hunter Biden. Independent fact-checkers at PolitiFact and The Washington Post have found no evidence to support the allegations, and Ukraine's prosecutor told Bloomberg News Service in May that he had no evidence of wrongdoing by Biden or his son.

In an interview on Fox News Sunday, Giuliani spun a word salad of allegations that was sometimes challenging to follow. He called the Biden story "a lot bigger than Spiro Agnew," a reference to the vice president who was forced to resign in 1973 for accepting bribes. Giuliani described Biden's son and business associates as "a recovering drug addict; [John] Kerry's stepson, and [mobster] Whitey Bulger's nephew." He declared: "I've got a nice straight case of Ukrainian collusion," mentioning money transfers and citing various Ukrainian officials. But he questioned whether the news media would investigate anything involving Biden. "They have been covering up for years, China, plagiarism," he said.

The strategy of aggressively going on offense seems familiar, too. In 2016, faced with allegations of personal misbehavior and Russian meddling, Trump and his supporters countered with questions about how Hillary Clinton, the former secretary of state, had handled the deadly attack on a U.S. outpost in Benghazi and whether she had mishandled her official emails.

These days, chants of "Lock Her Up" still sometimes ring through Trump rallies.