Proof by assertion is a common form of fallacious reasoning whereby a claim is repeated without supporting evidence. This is especially common in politics, and Congresswoman Omar's most recent controversy stems from her February 2019 statement that U.S. politicians defend and support Israel for financial support or, as she claims, "It's all about the Benjamins".
Beyond the obvious anti-semitic nature of Omar's comment, her claim is problematic because it is divisive, lacks proof, and serves as fodder to agitate and influence public perception. Statements like hers have no redeeming value in the marketplace of ideas and do nothing to promote substantive discussions about legislation and public policy. For legislative politics to be reduced to empty assertions, media attention, thoughtless accusations, social media clicks, and political popularity, this is fraudulent and a disservice to the ideals of representative democracy. Politics by assertion is nothing less than shameless political smear, and when politicians like Omar resort to scorched-earth politics to fuel acrimony, controversy, and celebrity, primarily at the expense of legislative success, America suffers and good government grieves.
On a more basic level, Omar's narrative is morally indefensible because truth is subordinated and compromised in order to propagate politically driven rhetoric about her opponent's political motives. Omar's assertion ascribes corruption and ill-intent to her opponents thereby creating a cloud of suspicion and an unfavorable impression. It is destructive through-and-through and is incompatible with the ideals liberals espouse, namely unity, tolerance, and civility.