WASHINGTON -- House Speaker Nancy Pelosi said Thursday the country faces a "constitutional crisis" because of President Donald Trump's resistance to congressional investigation, and she promised a methodical, if lengthy, effort to pursue oversight of the White House.

Pelosi made no promise for a swift House vote to hold Attorney General William Barr in contempt of Congress because of his refusal to release special counsel Robert Mueller's full report, deferring to a fresh subpoena for the document from the Intelligence Committee that's due back next week. She continued to tamp down talk of impeachment.

Instead, Pelosi foreshadowed a long-game strategy of Congress confronting a White House she suggested is all but goading her with its refusal to comply with oversight demands.

"The president thinks it is a laughing matter," Pelosi said. "This is about the American people and their right to know, and their election that is at stake -- and that a foreign government intervened in our election -- so we can prevent it from happening again.

"We won't go any faster than the facts take us or any slower."

The step-by-step approach has been Pelosi's touchstone for the escalating standoff between the two branches of government in the aftermath of Mueller's report. By rebuffing Congress, she said, the White House is essentially committing the kind of obstruction of justice Mueller probed in his report.

"The president is almost self-impeaching," she said.

The House Judiciary Committee voted Wednesday to hold Barr in contempt of Congress, capping a day of ever-deepening dispute between congressional Democrats and the Republican White House. Ahead of the vote, Trump for the first time invoked the principle of executive privilege, claiming the right to block lawmakers from the full report on Mueller's probe of Russian interference to help Trump in the 2016 election.

Committee Chairman Jerrold Nadler of New York declared the action by Trump's Justice Department a clear new sign of the president's "blanket defiance" of Congress' constitutional rights to conduct oversight.

"We did not relish doing this, but we have no choice," Nadler said after the vote.

The White House's blockade, he said, "is an attack on the ability of the American people to know what the executive branch is doing." He said, "This cannot be."

But Justice Department spokeswoman Kerri Kupec said it was disappointing that members of Congress "have chosen to engage in such inappropriate political theatrics."

Barr made "extraordinary efforts" to provide Congress and the public with information about Mueller's work, she said.

White House press secretary Sarah Sanders said neither the White House nor Barr "will comply with Chairman Nadler's unlawful and reckless demands."

Late Wednesday the chairman of the House Intelligence Committee issued his own subpoena to the Justice Department for the full Mueller report, as the confrontation intensified.

Democratic Rep. Adam Schiff of California, whose committee had previously requested the documents, said he had no choice but to compel the department's compliance. He warned that if it continues to "ignore or rejects our requests," the panel could take legal action.

Kupec declined to comment.

Though the White House initially hesitated on invoking privilege, Trump told his staff and political advisers in recent weeks to refuse to cooperate with Democrats, declaring the party's goal was simply to damage him politically going into his re-election campaign. The coming legal battle could stretch to 2020, and the White House is aiming to tie up congressional probes until Election Day.

Executive privilege is the president's power to keep information from the courts, Congress and the public to protect the confidentiality of the Oval Office decisionmaking process.

The president's decision was weeks in the making, the next inevitable escalation between the White House and Congress because of a number of probes. The White House has rejected all efforts to probe Trump's business dealings or tax returns as well as the West Wing's security clearance procedure.

Democrats made their case that Congress was at a historic juncture as it confronts what they consider Trump's stonewalling of lawmakers' ability to conduct oversight. Republicans portrayed the majority as angry and lashing out at Barr after the special counsel did not find that Trump colluded with Russia to swing the 2016 election.

Said Democrat Sheila Jackson Lee of Texas: "The president now seeks to take a wrecking ball to the Constitution of the United States."

But the panel's top Republican, Doug Collins of Georgia, said Democrats were manufacturing a crisis and rushing the process to "sully Bill Barr's good name."

If the contempt citation is approved by the House, where the Democrats hold a solid majority, it would almost certainly move to an unusual, and potentially protracted, multi-pronged court battle with the Trump administration.

The contempt finding could be referred to the U.S. attorney for the District of Columbia, a Justice Department official who would be likely to defend rather than oppose Barr. Democratic House leaders could also file a lawsuit, though the case could take months or even years to resolve. Some committee members have suggested they also could fine Barr as he withholds information.

In a letter Wednesday to Trump, Barr explained the special counsel's files contain millions of pages of classified and unclassified information. He said it was the committee's "abrupt resort to a contempt vote" that "has not allowed sufficient time for you to consider fully whether to make a conclusive assertion of executive privilege."

Barr told Trump he should assert privilege now, "pending a full decision on the matter."

Barr released a redacted version of Mueller's 400-plus-page report to the public last month, but Democrats subpoenaed the full document , along with underlying evidence.

Mueller, in his report, said he could not establish a criminal conspiracy between the Trump campaign and Russia, but he did not reach a conclusion on whether Trump obstructed justice. Barr said he and Deputy Attorney General Rod Rosenstein decided there were not grounds to charge Trump with obstruction.

Lemire reported from New York. Mike Balsamo and Laurie Kellman of the Associated Press in Washington contributed.