The time has come for Congress to launch an impeachment investigation of President Donald Trump for obstruction of justice.

The remedy of impeachment was designed to create a last-resort mechanism for preserving our constitutional system. It operates by removing executive-branch officials who have so abused power through what the framers called "high crimes and misdemeanors" that they cannot be trusted to continue in office.

No American president has ever been removed for such abuses, although Andrew Johnson was impeached and came within a single vote of being convicted by the Senate and removed, and Richard Nixon resigned to avoid that fate.

Now the country is faced with a president whose conduct strongly suggests that he poses a danger to our system of government.

Ample reasons existed to worry about this president, and to ponder the extraordinary remedy of impeachment, even before he fired FBI Director James Comey and shockingly admitted on national television that the action was provoked by the FBI's intensifying investigation into his campaign's ties with Russia.

Even without getting to the bottom of what Trump dismissed as "this Russia thing," impeachable offenses could theoretically have been charged from the outset of this presidency. One important example is Trump's brazen defiance of the foreign emoluments clause, which is designed to prevent foreign powers from pressuring U.S. officials to stray from undivided loyalty to the United States. Political reality made impeachment and removal on that and other grounds seem premature.

No longer. To wait for the results of the multiple investigations underway is to risk tying our nation's fate to the whims of an authoritarian leader.

Comey's summary firing will not stop the inquiry, yet it represented an obvious effort to interfere with a probe involving national security matters vastly more serious than the "third-rate burglary" that Nixon tried to cover up in Watergate. The question of Russian interference in the presidential election and possible collusion with the Trump campaign go to the heart of our system and ability to conduct free and fair elections.

Consider, too, how Trump embroiled Deputy Attorney General Rod Rosenstein and Attorney General Jeff Sessions, despite Sessions' recusal from involvement in the Russia investigation, in preparing admittedly phony justifications for the firing on which Trump had already decided. Consider how Trump used the vice president and White House staff to propagate a set of blatant untruths - before giving an interview to NBC's Lester Holt that exposed his true motivation.

Trump accompanied that confession with self-serving - and manifestly false - assertions about having been assured by Comey that Trump himself was not under investigation. By Trump's own account, he asked Comey about his investigative status even as he was conducting the equivalent of a job interview in which Comey sought to retain his position as director.

Further reporting suggests that the encounter was even more sinister, with Trump insisting that Comey pledge "loyalty" to him in order to retain his job. Publicly saying he saw nothing wrong with demanding such loyalty, the president turned to Twitter with a none-too-subtle threat that Comey would regret any decision to disseminate his version of his conversations with Trump - something that Comey has every right, and indeed a civic duty, to do.

To say that this does not in itself rise to the level of "obstruction of justice" is to empty that concept of all meaning. Obstruction of justice was the first count in the articles of impeachment against Nixon and, years later, a countagainst Bill Clinton. In Clinton's case, the ostensible obstruction consisted solely in lying under oath about a sordid sexual affair that may have sullied the Oval Office but involved no abuse of presidential power as such.

But in Nixon's case, the list of actions that together were deemed to constitute impeachable obstruction reads like a forecast of what Trump would do decades later - making misleading statements to, or withholding material evidence from, federal investigators or other federal employees; trying to interfere with FBI or congressional investigations; trying to break through the FBI's shield surrounding ongoing criminal investigations; dangling carrots in front of people who might otherwise pose trouble for one's hold on power.

It will require serious commitment to constitutional principle, and courageous willingness to put devotion to the national interest above self- interest and party loyalty, for a Congress of the president's own party to initiate an impeachment inquiry. It would be a terrible shame if only the mounting prospect of being voted out of office in November 2018 would sufficiently concentrate the minds of representatives and senators today.

But whether it is devotion to principle or hunger for political survival that puts the prospect of impeachment and removal on the table, the crucial thing is that the prospect now be taken seriously, that the machinery of removal be reactivated, and that the need to use it become the focus of political discourse going into 2018.