One doesn't require deliberate back room cooperation with Russia to be a Russian asset. Your actions and what you say can make you one indirectly. It doesn't require knowing cooperation.
"The DNC and the corporate media are rigging the election again," the Hawaii congresswoman claimed.
www.newsweek.com
Her campaign is deliberately engaging in tactics and behavior that go beyond attacking a single candidate; her threatening a boycott of the Democratic debate and also her apparent grievances beforehand cast doubt on the fairness and legitimacy of the political process, which is what Putin wants even more than one particular candidate winning or losing.
Having an asset as President is bad, but it's not even the worst of what Putin can do to the US. He can cast doubt on the entire political system's fairness. Right now, the United States is a formidable nation that has national interests. It has fewer national interests when people can't agree on who legitimately runs the country, which makes it impossible for people to agree on what our national interests are.
She can be called a Russian asset because she's suggesting that the political system is treating her unfairly and threatening to boycott the debates.
She knows - or should know as a former veteran - that her behavior - things she says, potentially undermines confidence in the system, which is what Putin and the Russians would want from an asset.