"The FBI speculates that the identified U.S. person was likely looking for information that could be used to discredit Candidate #1's campaign.
. . .
Notwithstanding Source #1's reason for conducting the research into Candidate #1's ties to Russia, based on Source #1's previous reporting history with the FBI, whereby Source #1 provided reliable information to the FBI, the FBI believes Source #1's reporting herein to be credible."

but they didnt disclose the political oppo source
one issue is that they cite to the September 23 article (for which we have learned, iirc, that Steele was the source to) as an additional source, although that is clearly redundant with source #1.
but then that is followed by about 6 pages of reactions, BEFORE getting to their conclusion that they have probable cause. so unless those 6 pages are incoherent rambling, its irresponsible to say "omg that's all they relied on"