Finally, the dam breaks.

For weeks, Republicans in Congress and the Trump administration have resisted growing calls to appoint a special prosecutor to oversee FBI investigations into Russian tampering in the November election. Now on Wednesday, Rod J. Rosenstein, the widely respected deputy attorney general, appointed Robert Mueller III, the widely respected former head of the FBI, to be that special prosecutor.

OPINION

Mueller’s appointment restores a degree of confidence that a deep and honest probe will be conducted into Russian hacking in the election and into any inappropriate contact between Russia and the Trump campaign. A House probe and two separate Senate investigations have been puttering along at quarter speed because unenthusiastic Republican leaders would rather the whole business just go away.

Mueller’s appointment also goes a ways toward shoring up confidence in the integrity of our nation’s system of governance, which requires that men and women of principle put their country first.

The betting was always strong in Washington that Rosenstein would find a way to do the “right thing” — resign or call the boss out or appoint a special prosecutor — after Trump falsely claimed last week that it was Rosenstein who had initiated the firing of FBI Director James Comey. Rosenstein had written a memo highly critical of Comey, but had not called for his firing. That was all Trump.

Democrats would be smart to take a breath now and let Mueller do his job. Talk of impeachment was premature before Mueller’s appointment and is more so now. Let’s first get to the bottom of what the president has dismissed as this “Russian thing.”

Republicans, for their part, might be smart to look to Rosenstein to see how to show a little spine in a moment of crisis. Rosenstein appointed a special prosecutor despite Trump’s clear preference that he not do so, and it is likely that Rosenstein’s immediate boss, Attorney General Jeff Sessions, also was not keen on the idea.

When Rosenstein decided to appoint Mueller, you could argue he was looking out for himself. His reputation took a hit when Trump roped him into the Comey firing fiasco. But given the quality of the appointment, there is no doubt he was looking out for his country.

Mueller is highly respected by both Democrats and Republicans, and he has served presidents of both parties. He was director of the FBI from 2001 to 2013, appointed first by President George W. Bush and then by President Barack Obama. The Senate confirmed his nomination unanimously both times.

Now the real investigation into this “Russian thing” begins.