Former U.S. Vice President Joe Biden, 2020 Democratic presidential candidate, speaks at the Teamsters Local 249 hall during a campaign stop in Pittsburgh, Pennsylvania, U.S., on Monday, April 29, 2019.

Democratic 2020 frontrunner Joe Biden said Congress will have "no alternative" but pursuing impeachment if President Donald Trump or his associates block investigations based on special counsel Robert Mueller's Russia report.

In an interview with ABC's "Good Morning America" broadcast Tuesday, Biden said that Mueller's 448-page report on Russian election interference included "about seven or eight things that are left undone."

"The Congress is attempting to take that up" for investigation as they should be doing, said Biden, who launched his presidential campaign last week. "And if in fact [Trump or his associates] block the investigation, they have no alternative but to go to the only other constitutional resort they have," which Biden said "is impeachment."

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Biden has taken aim squarely at Trump since announcing his candidacy, and now has established his own line in the sand on impeachment. But he said in the ABC interview that "my job in the meantime is to make sure he's not back as president of the United States of America."

The White House did not immediately provide a comment on Biden's remarks.