From your link:
""We were never going to find a contract signed in blood saying, 'Hey Vlad, we're going to collude,'" one Democratic aide said."
"The series of contacts between Trump's associates, his campaign officials, his children and various Russians suggest a campaign willing to accept help from a foreign adversary, the Democrats say."
"By many counts, Trump and his associates had more than 100 contacts with Russians before the January 2017 presidential inauguration."
As we now know, Trump's people were negotiating with Russia for a major real estate project in Moscow at the same time that Trump was claiming to have no dealings with Russia, and while he was softening the Republican party's platform regarding Ukraine before the Republican convention.
The Republican Senate doesn't think there is direct, iron-clad evidence of collusion between the Trump campaign and Russia. Mueller may reach a different conclusion. Besides, I and many others always assumed the best bet for finding illegal conflicts of interest between Trump and the Russians is by following the money.