The chairman of the Democratic National Committee is laying into his own party, accusing it of not doing enough to confront what he sees as the threat to American democracy posed by President Trump.

“Seven months in and Trump is openly scheming to rig the midterm map and cling to power, plotting his way towards an unconstitutional third term,” Ken Martin told fellow Democrats at the party’s summer meeting at Minneapolis on Monday.

“This is not politics as usual,” Mr. Martin said, adding that Democrats “are the only thing standing in” the president’s way.

“I’m sick and tired of this Democratic Party bringing a pencil to a knife fight,” Mr. Martin said. “We cannot be the only party that plays by the rules anymore. We’ve got to stand up and fight.”

Mr. Trump has brushed off suggestions that he might seek a third term, something prohibited by the Constitution, but has not ruled it out. Asked on CNBC’s “Squawk Box” earlier this month whether he would seek a third term, he said, “Probably not. I’d like to.”

Mr. Martin is trying to rally a party that got drubbed in last year’s elections and faces plunging voter registrations ahead of next year’s midterm elections.

The party not in control of the White House traditionally picks up seats in Congress in midterm elections, and Democrats want to flip control of the House, where they need to gain only five seats to take control.

Mr. Trump has urged Republican governors to redraw the congressional districts in their states to help protect the Republican majority.

Texas has already approved a plan that is expected to net five seats for Republicans. California is countering with its own plan to tip five Republican seats to Democrats, but that plan must be approved by voters in a special election this November.

This is the first official Democratic National Committee meeting since Mr. Martin became the chairman. He says the party has launched a 10-year program to win elections at all levels of government, but there are pressing problems.

The committee had only $14 million in the bank in July compared with $84 million for the Republican National Committee, the Associated Press reported.

Mr. Martin said the party needs to get back to a message that resonates with voters and donors. The biggest opening appears to be the economy, and on Monday, Mr. Martin claimed that Mr. Trump is hurting American families with his tariff policy.

Democrats have a shrinking window before next year’s elections to try to frame a message around the economy that will unite a fractured party and make inroads with voters who have drifted to the Republican Party.