Democrats have swept the first major elections of Donald Trump's second term, with the US president saying the ongoing government shutdown is a "big factor".

Democrats dominated the first major Election Day since President Donald Trump returned to the White House, and there are signs that the economy — specifically, Trump’s inability to deliver the economic turnaround he promised last fall — may be a real problem for Trump’s party.

Trump's campaign-style speech to business leaders in Miami was designed to tout his economic accomplishments as voters increasingly say his administration is not doing enough.

U.S. President Donald Trump's campaign on Thursday sued Philadelphia County's Board of Elections, seeking an emergency injunction barring it from counting ballots so long as Republican observers are not present as it said was required under Pennsylvania law.