In the heated discussions over the effects of fake news on democracy and civil society, Donald J. Trump has often taken center stage.

He has used false claims to attack his political opponents, question the legitimacy and loyalty of the Obama administration and other Democrats, and undermine the news media, the federal government and other institutions that many of his supporters do not trust.

The practice has paralleled his rise from reality TV star to holder of the nation’s highest elected office, according to an analysis of his social media activity.

When discussing some of his claims, Mr. Trump has cited as evidence articles posted through Breitbart News, manipulated YouTube videos and celebrity gossip publications like The National Enquirer.

Mr. Trump has also tweeted links from right-wing blogs like WND.com and TheRightScoop.com that often promote sensational conspiracy theories and contain little original reporting.

His sourcing highlights the bounty of misinformation accessible on the web and its power in a deeply divided America — especially when endorsed by someone of Mr. Trump’s influence and visibility.

He offered this explanation for his actions while discussing an altered YouTube video he had tweeted as part of an unsubstantiated claim that a protester at one of his rallies had ties to the Islamic State: “I don’t know what they made up; all I can do is play what’s there,” Mr. Trump said on NBC’s “Meet the Press.”

“All I know is what’s on the internet.”

Below are examples from the last several years of Mr. Trump’s penchant for making fraudulent claims and backing them up with information gleaned from unsubstantiated sources.