President Trump has kicked off his general election advertising campaign with a xenophobic attack ad against Joseph R. Biden Jr., the presumptive Democratic nominee, the opening shot in a messaging war that is expected to be exceptionally ugly.

In a minute-long digital ad released late Thursday that relies heavily on imagery of China and people of Asian descent, the Trump campaign signaled the lines of attack it will use in its attempts to rally the president’s base and define Mr. Biden. The ad reprises accusations Mr. Trump has made that the former vice president’s family profited from his relationships with Chinese officials and presents selectively edited scenes and statements attempting to portray him as doddering and weak.

For the president and his allies, the approach represents their assessment of the race as it narrows into a one-on-one contest with Mr. Biden, the opponent who is least susceptible to their charges that the Democratic Party is too far outside the political mainstream.

The new ad also shows that while the country has changed drastically in recent weeks amid a national health crisis, the president has not. He continues to lead the nation and run his campaign the way he always has: by belittling his adversaries and exploiting racial discord.