The former 'Daily Show' host was one of several people in attendance at the White House to watch 46 sign the PACT Act into law ... and you could tell he was emotional to see it reach the finish line after a hard-fought battle. The Prez made sure to acknowledge his good work, saying the bill wouldn't be before him if it wasn't for everything he'd done.

JS took it all in humbly, and throughout the ceremony ... was also hugging vets who were able to make it in person, rubbing shoulders with politicians in the room, and getting pats on the back by pretty much everybody around. It's well deserved.

As we all saw over these past couple weeks, Jon was on the front lines in getting U.S. Senators to vote for PACT -- even after many of them, mostly Republicans, flip-flopped at crucial points in the legislative process ... nearly torpedoing it all together.

There was a lot of handwringing over verbiage -- including claims that Dems were trying to play a budgetary game in terms of the type of spending this would fall into long-term -- but Jon always stuck to his guns and called out misinformation left and right in a variety of ways.

He spoke directly to reporters in very impassioned terms a few times on Capitol Hill ... and even got into some shouting matches with people he considered trolls acting in bad faith.

Of course, Jon also used social media pretty creatively -- and in the end, it paid off. Just goes to show ... the guy knows how to get stuff done in D.C. when he puts his mind to it.