The elongated broadsides on 20 Metro buses feature a photo of the Nazi German dictator in conversation with "his staunch ally" Haj Amin al-Husseini, grand mufti of Jerusalem during World War II.
"Islamic Jew-hatred: It's in the Quran. Two-thirds of all US aid goes to Islamic countries. Stop racism. End all aid to Islamic countries," the ad states, over a fine-print disclaimer from the Metro Transit Authority.
The ads, which are to run until mid-June, were placed by the American Freedom Defense Initiative (AFDI), which aims to "raise awareness of the depredations of Islamic supremacism", according to its website.
It hopes the campaign will raise $20,000 by Friday via an online crowd-funding campaign that, as of Tuesday, had yielded about $7,500.
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"We're not able to refuse ads on the basis of content," a spokeswoman for Metro told AFP, citing a 2012 court case that allowed another AFDI bus ad on the grounds that it was free speech.
On its website, AFDI co-founder Pamela Geller called the campaign a direct response to like-sized Washington bus ads placed in April by American Muslims for Palestine which read: "Stop US aid to Israel's occupation."
As Muslim leader in then British-ruled Palestine, Husseini sought Hitler's support for an Arab and Muslim homeland that would be free of Jews.
Council on American-Islamic Relations (CAIR) spokesman Ibrahim Hooper said that such "inflammatory" ads were clearly intended "to promote hatred of Islam and Muslims".
He told AFP that CAIR is developing on its own bus ads "to promote mutual understanding as a response to Geller's hate ad". In the meantime, he added, it's giving away free Korans.