HELSINKI—President Donald Trump, standing beside Russian leader Vladimir Putin, questioned the U.S. intelligence conclusion that Moscow meddled in the 2016 election, a move lawmakers of both parties said was a stunning alignment with an adversary.

Mr. Trump said he and the Russian president “spent a great deal of time” discussing the matter during their four hours of talks here on Monday, and said Mr. Putin “was extremely strong and powerful in his denial.”

Asked...