The 38-year-old California representative never garnered more than one percent support in polls, according to Real Clear Politics -- though he got a slight boost in the June 26-27 party debates when he suggested that frontrunner Joe Biden, the 76-year-old former vice president, "pass the torch" to a younger generation.
"Today ends our presidential campaign. But it is the beginning of an opportunity in Congress, with a new perspective shaped by the lives that have touched mine and our campaign throughout these last few months, to bring that promise of America to all Americans," Swalwell said.
His withdrawal leaves more than 20 candidates still in the race to be the party's choice to battle Republican President Donald Trump in the 2020 election.
Biden continues to lead the race, but since the debates his share of support has sagged to 26 percent, down from 32 percent, according to Real Clear Politics.
Meanwhile, California Senator Kamala Harris has surged into second, helped by her debate attack on Biden in a back-and-forth on racial discrimination.
Harris jumped from seven percent to 15.2 percent, while previous second-place holder Senator Bernie Sanders, the 77-year-old leftist, sagged to 14 percent. In close fourth place was Senator Elizabeth Warren, having steadily risen over the past month to 13.8 percent support.
Warren, who also turned in a solid debate performance, announced Monday that her presidential campaign had pulled in USD 19.1 million in the second quarter, topping both Sanders and Harris and slightly less than Biden, despite Warren eschewing fundraisers with deep-pocketed donors.
Biden brought in USD 21.5 million in the April-June period, while Harris garnered only USD 12 million and Sanders USD 18 million.
Warren, who has sought to avoid relying on large donations from wealthy business, reached the USD 19 million threshold on donations averaging just USD 28 each.
"I am humbled by the depth of grassroots commitment to our campaign," Warren tweeted.
"This is how we make our government and democracy work for everyone, not just the wealthy and well-connected.
Thank you, and let's keep at it." Warren's take was nevertheless well behind that of Peter Buttigieg, the young gay mayor of South Bend, Indiana, who drew a surprisingly strong USD 24.8 million in the second quarter.
But Buttigieg remains a distant sixth place in the polls with just 5.2 percent support. Meanwhile Trump has reported reaping USD 105 million in donations to his reelection campaign in the second quarter.
The next debates, planned over two nights at the end of July, could accelerate the thinning of the Democratic field. The first real contest between the candidates will take place on February 3, 2020, in the Midwest state of Iowa.