- Trade uncertainty is the "800-pound gorilla keeping a lid on prices now." says Siegel, who for years correctly called the march higher on Wall Street.
- However, he points to some other headwinds including a "still pretty aggressive" Fed and the November midterm elections.
The stock market could increase another 10 percent this year if the U.S. were to settle its trade war with China and reach a new North American Free Trade Agreement, Jeremy Siegel told CNBC on Thursday.
The was already 7 percent higher for the year as of Wednesday's close.
"The next few months are going to be a challenge," said Siegel, the Wharton School finance professor who for years correctly called the march higher on Wall Street. He recently became a bit more cautious.
"But if we can get China [and] NAFTA settled, I see a 10 percent pop in the market because I think that's the kind of 800-pound gorilla keeping a lid on prices now," Siegel said on "Squawk Box."
On Tuesday, the S&P 500 during the trading session hit an all-time high for the first time in seven months but failed to close at a record. A day later, the stock bull market became the longest ever on record since World War II. It started March 9, 2009.
In the beginning of 2009, Siegel was one of the first forecasters to express optimism about stocks coming out of the financial crisis, which tanked markets.
"When we go back all the way to 2009, I thought that was one of the easiest calls I made," Siegel said Thursday on CNBC.
"The previous longest bull market ended in March of 2000 with a price-earnings ratio at 30 for the S&P. We're looking at 18 right now for looking at this year's earnings," he said, contending the current market is "much less overvalued."
Interest rates are also much lower now than they were after the dot-com bubble burst in 2000, he argued, saying the last bull market and this one feel nothing like one another.
Siegel said a market multiple of 18 to 20 times earnings in a "low rate environment with a stable economy" is not too lofty.
However, he did point out some headwinds including trade uncertainty, a Federal Reserve that "looks still pretty aggressive" and the midterm elections.
"Oddsmakers are saying that maybe Democrats are going to take the House," Siegel observed.
If Republicans were to lose control of the House but keep the Senate, that might not be the worst outcome for markets because President Donald Trump's corporate tax cuts and deregulation are already in place, Siegel said.
Without control of the House and Senate, Democrats would not be able to rollback Trump's economic agenda, he said.
"The most important plus of the Trump adminstration was that corporate tax reform," Siegel said. "That really was a good 10 [or] 12 percent of the earnings growth. Two-thirds of earnings growth this year was due to that."
Siegel said it's pretty hard for Democrats to run against low unemployment and a strong economy. Despite expectations for a tough November for Republicans at the polls, those factors are positives for GOP candidates, he concluded.