"This election is Donald Trump's to lose," Forbes told CNBC's "Squawk Box" on Monday. He said Americans don't really want to vote for Clinton because her presidency would effectively be a "third term" of President Barack Obama.
"But [Trump] has got to put out credible alternatives, ... otherwise [Clinton] will win by default," said the chairman and editor-in-chief of Forbes Media, who also sees third-party candidates, particularly Gary Johnson of the Libertarian party, as a wildcard.
In the latest RealClearPolitics polling aggregator, Johnson, a former two-term Republican governor of New Mexico, has 7.8 percent support nationally, while the Green Party's Jill Stein, a longtime environmental activist, has 4.8 percent.
Meanwhile, Clinton's lead has widened to an average of 42.3 percent to Trump's 36.7 percent, with all four candidates polled together.
Despite some high-profile dissenters, Forbes said: "The bases of the parties are starting to return to the candidates, even though both are unpopular with many sectors."
The latest revolt came from Hank Paulson, who served as Treasury secretary during the presidency of George W. Bush and in the thick of the 2008 financial crisis.
Paulson wrote in a Washington Postop-ed Friday that he's going to vote for Clinton. The former Goldman Sachs chief called Trump's rise to presumptive GOP nominee a "populist hijacking" of one of the great political parties of the United States.
Forbes, who has endorsed Trump, blamed the New York tycoon's recent troubles on the campaign's inability to control the political narrative. "Part of Trump's campaign problems is his communications department, of which there isn't [one]," he said.