"What's in it for Grassley? It's hard to find anything. He was just re-elected in 2016 to his seventh term and he's 84. He also comes from Iowa where the polls say President Trump's support has very much eroded."

Grassley even tweeted an emphatic reminder to Comey Wednesday night about how federal law enforcement agencies must remain under the oversight of elected officials.

This is a fabulous development for the Trump team because it now has a respected and powerful ally joining it in its criticism and suspicions about Comey, the Justice Department and the FBI.

The investigation of potential crimes committed by Washington bureaucrats in alleged attempts to thwart the Trump campaign may come to nothing. But Grassley's depiction of this effort as being a defense of the ultimate sovereignty of the voters now gives it a higher calling of sorts. This will help Grassley's Senate investigation efforts and those of House Intelligence Committee chairman Devin Nunes.

It's important to note here that Grassley has already been perhaps the most effective ally for President Trump since he took office. It was Grassley who cleared away the last vestiges of Democratic Party obstructionism against the Trump judicial nominees back in November. That move helped President Trump set the record for most federal judges appointed in the first year of a presidency. No matter how long Donald Trump remains as president, the continuing pace of judicial nominations coming out of the White House will have a very long lasting and important impact. Without Grassley, that simply doesn't happen.

But Grassley is not a Trump toady. First off, he took his time in his decision to eventually put the Trump judicial nominees on the fast track.

Second, Grassley handled another incident last month with a prudent level of independence. When Trump critics were rightfully questioning some of his judicial nominees who did poorly in their congressional confirmation hearings, Grassley didn't rush to defend the nominations.

And he did clearly and publicly tell the White House to rescind the nominations of the two most egregiously embarrassing judicial nominees.

Those nominees did step aside, and there wasn't a nasty Trump tweet or anything else in response. Grassley literally forced the Trump team to adjust course, and he did it without starting World War III.

What's in it for Grassley? It's hard to find anything. He was just re-elected in 2016 to his seventh term and he's 84. He also comes from Iowa where the polls say President Trump's support has very much eroded. That means his support for the president could make the next year or so a nightmare back home. Finally, the hint of disastrous scandal that swirls around the Trump team could stain Grassley's reputation by association.

But all of the above just adds to Grassley's value for the White House.

He doesn't have to help Trump in these endeavors, but he is. He has more to lose than to gain, but he persists and now with the FBI scrutiny, he's taking it to a new level. The best explanation for this seems to be that as Judiciary Committee chairman, he wants to maintain as much of the power of his office as possible over unelected bureaucrats and rank-and-file senators.

Compared to the Bannons, Scaramuccis, and even Attorney General Jeff Sessions, Grassley has done more to help President Trump than anyone actually in his administration.

Now the only question is: How long will it last? This is one working relationship this president cannot afford to mess up.

Commentary by Jake Novak, CNBC.com senior columnist. Follow him on Twitter @jakejakeny.

For more insight from CNBC contributors, follow @CNBCopinion on Twitter.