Kilimnik, internal polling data. Now, in order to PROVE collusion, you'd need evidence of what happened to that material after Mr. Kilimnik received it...and evidence that Mr. Manafort's actions were known and acknowledged by Mr. Trump...sadly, we don't have that information due to a lack of cooperation on the part of anyone associated with the Trump campaign. and are left with only questions...questions that needed to be added to others, like...why did DJT Jr. lie about a meeting with a well placed Russian attorney and exclaim he'd "love" to receive from her, compromising material regarding Hillary Clinton, especially if just before the election.
Hopefully, you get the picture...there's every reason to believe that the Trump campaign colluded with Russia in advance of the 2016 election...and every reason to believe that such behavior would continue...to deny this is naivete at its worst...and our Intelligence Services are anything but naive, so it's no surprise that they would continue to surveil Donald Trump.
As an example, here's a personal story...even a young engineer like myself, with a Top Secret "Q" clearance working on one of the DOD's most secret projects, was subject to surveillance whenever traveling outside the U.S....so based on that, I'd be shocked and dismayed if given the Manafort/Kilimnik revelation, a candidate for Commander-in-Chief of ALL U.S. secrets was not surveilled as well...wouldn't you as an American citizen feel the same way?