This is a very complex issue and I think it's important to be careful with your words here. Meddling is not the same as colluding, or cooperating. And it definitely is not the same as "undermining democracy" so keep in mind if you argue with someone about this that you will need to agree on terms first.
There is also a huge gray area here if we're talking about legality. Direct interference (tampering with ballots) and monetary donations are prohibited. Though monetary donations from foreign governments are common, they are typically made indirectly. Other things that are alleged to have happened in this election simply have no precedent. Should the investigation reach a courtroom, the legality of these things will likely be determined there.
Russia meddled in the 2016 election
Putin has been very clear about his preference in the election.
Many high ranking, respected government officials have taken this stance. Including very respected Republicans.
Trump did meet with a Russian official during his candidacy.
Trump Jr. emails prove that another Russian official was actively giving them information
Timing of James Comey firing is extremely suspect
Hillary campaign repeatedly rocked by advanced cyberattacks and leaks. This did not happen to Trump.
ODNI Report indicated with a high degree of confidence that this is the case and can be read here: https://www.dni.gov/files/documents/ICA_2017_01.pdf
The fact that most intelligence officials agree that this is the case, but haven't published any concrete evidence is indicative of the fact that some sort of classified evidence exists. It's possible that if information does exist, it was obtained illegally and can't hold up in court.
Russia did not meddle in the 2016 election