I don't assume that, and for various reasons. But, that assumption might hold, too.
I think Trump was under surveillance before Carter Page and Papadopolous joined the campaign. The surveillance has NO pretext. It was done as a matter of seeking advantage where ever it could be found. At some point before Page and Papadopolous were identified, Trump's opposition chose "Russian collusion" as a viable false accusation. There are multiple reasons for this: Trump had conducted Miss Universe, sought deals in Russia; opposition wants to increase tension with Russia for geopolitical purposes.
I think Trump's campaign selected Page and Papadopolous, without them being pushed forward by team Obama, team Comey, or whatever label attaches to the opposing force.
I think Page and Papadopolous were targeted as capable of being "tainted" as "Russian links" - and this was AFTER the Russian collusion scheme" was hatched - by review of list of names that team Trump submitted to WaPo as foreign advisers. See "Byron York: When did Trump-Russia probe begin?."
>>I think Trump was under surveillance before Carter Page and Papadopolous joined the campaign. <<
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I agree with this. The whole story strikes me as after-the-fact trying to establish a veneer of legality.
Maybe that was the insurance policy they were concerned with ... to have a line of defense (against personal prosecution) in the unlikely event Trump were to win.