This is somewhat disingenuous. I doubt very many Americans think Trump is a sleeper
agent in the literal sense of that term.
There is, however, considerable circumstantial evidence that he may be compromised by the Russians. The fact that Trump has been less than wholly forthcoming regarding his connections with Russia has only deepened this concern.
Multiple people in the Trump orbit--including Trump himself--have had repeated communications with Putin, Russian oligarchs, or their proxies, and have gone to considerable lengths (such as by conducting meetings in private and confiscating the translator's notes) to hide the contents of these communications from official records and lawfully constituted legal investigations.
Given that Trump is believed to be deeply in debt (see the recent NYT series on Trump's tax records), has had business dealings inside Russia (e.g. Trump Tower Moscow), and has benefited from Russian state-sponsored criminal hacking[0], hidden communications between the Trump orbit and Russia are cause for grave concern over whether Trump has inappropriate ties to the Russian government.
Read the Mueller Report and pay particular attention to what could not be proven because the necessary evidence had been destroyed or was otherwise inaccessible.[1]
Unlike the current flavor of Republican conspiracy theorizing, the Trump-Russia story is not a bunch of lunatics deluding themselves into thinking that the Democratic party is keeping children captive in the basement of a pizza restaurant. It's something backed by actual evidence and has lead to multiple prosecutions.
[0] https://www.washingtonpost.com/politics/2019/05/15/what-happ...
[1] https://en.wikipedia.org/wiki/Crossfire_Hurricane_(FBI_inves...