The case for impeachment is more of a political matter than a legal one, but let's start here:

If Donald Trump is permitted to interfere with an investigation into Russian meddling of the 2016 election, it's hard to fathom how our democracy would recover.

So with the Trump's allies now getting indicted three at a time, Congress must be alert to the possibility that this president -- who has impulse control of a toddler -- might find the temptation to intervene irresistible, and spell out some ground rules for presidential behavior as Special Counsel Robert Mueller's investigation goes forward.

Because any breach of these red lines should merit impeachment:

First, Congressional leaders must affirm that Trump allows the investigation to run its course. Remind him that firing Mueller is to repeat the mistake made by President Nixon, who fired Watergate prosecutor Archibald Cox in October 1973. Ten months later, with bloodhounds closing in, he resigned.

A second non-negotiable is to tell the president that he cannot defang Mueller's authority by using his pardon power to sabotage this investigation.

There is too much at stake to acquiesce on either matter.

Because the facts, surreal as they may be, are these: Trump appointed two foreign agents -- Manafort and Michael Flynn -- as his campaign chair and national security advisor.

Both his son and his foreign affairs advisor -- the hopelessly overmatched Donald Trump Jr. and the suddenly-infamous George Papadopoulos -- attempted to collude with a foreign adversary to fix the 2016 election, the success of which has yet to be determined.

And the chairman of the Trump campaign, Manafort, has been charged with tax fraud, money laundering, and conspiracy against the United States.

The president's response to the latest event was a predictable torrent of juvenile tweets, including a claim that there was "NO COLLUSION!" Regardless, the more layers Mueller peels away, the more vigilant Congress must be in preventing Trump's disruption of the inquiry, which he still pretends is a hoax.

Clearly, we're going to need a few patriots to keep him from turning this into a banana republic, and we've run short of those lately. House Speaker Paul Ryan responded to the indictments with a strategic shrug, because as he saw it, "I really don't have anything to add, other than nothing is going to derail what we're working on in Congress, because we're working on solving people's problems." Senate Majority Leader Mitch McConnell also went into a shell Monday.

Profiles in courage. If the chairman of Hillary Clinton's campaign was charged with 12 counts -- or her foreign advisor pled guilty to lying to the FBI about his relationships with foreign nationals -- we're fairly certain that the Republican leaders might say something about it.

If a President Clinton fired an FBI Director to shut down another probe -- as Trump did with James Comey in May -- she would have been impeached.

But they must say this to Trump: The lines have been drawn, the die has been cast, and Mueller -- an unassailable investigator -- must not be deterred.

Princeton historian Julian Zelizer put it this way: "He needs to play this out," he said of Trump. "Any interference with the investigation at this point is clearly a danger for him. Even if his campaign is totally innocent, we're at a point where he needs to have the investigation show it. Because trying to stop it is problematic."

That doesn't mean Republican leaders wouldn't rather peddle a bogus, ginned-up scandal involving Clinton selling uranium to Russians -- the House even launched an investigation last week with Ryan's blessing.

But removing Mueller must be the deal-breaker: He is just getting started, and you get the distinct impression that this president is starting to hear the branch creak.

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