Donald Trump started his Labor Day holiday with an attack on a top union leader.

Responding to criticism from the AFL-CIO president, Richard Trumka, the president tweeted that he “represented his union poorly on television this weekend”.

He added: “It is easy to see why unions are doing so poorly. A Dem!”

Later on a hot and steamy Monday morning, after seemingly abandoning a trip to his golf course in Virginia, Trump repeated the tweet with a spelling mistake corrected.

Trumka appeared on Fox News Sunday, where he said efforts to overhaul the North American Free Trade Agreement (Nafta) should include Canada.

Trumka, whose organization is an umbrella group for most unions, said the economies of the US, Canada and Mexico were “integrated” and “it’s pretty hard to see how that would work without having Canada in the deal”.

On Twitter on Saturday, Trump said there was “no political necessity” to keep Canada in Nafta. But it is questionable whether Trump can unilaterally exclude Canada from a deal to replace the three-country agreement without the approval of Congress. Any such move would probably face lengthy legal and congressional challenges – several Republicans in the closely divided Senate are insisting that a revised deal include Canada.

Negotiations to keep Canada in the trade bloc are to resume this week as Washington and Ottawa try to break a deadlock over issues such as Canada’s dairy market and US efforts to shield drug companies from generic competition.

Trump wants to get a trade deal finalized by 1 December.

Trumka also said of Trump: “The things that he’s done to hurt workers outpace what he’s done to help workers.”

He argued that Trump had not come through with an infrastructure program and had overturned regulations, which “will hurt us on the job”.

Asked about the low unemployment rate and economic growth, Trumka said: “Those are good, but wages have been down since the first of the year. Gas prices have been up since the first of the year. So, overall, workers aren’t doing as well.”

On Monday, Trump touted the economy, saying: “Our country is doing better than ever before with unemployment setting record lows.” He added: “The Worker in America is doing better than ever before. Celebrate Labor Day!”

The unemployment rate of 3.9% is not at the best point ever – but it is near the lowest in 18 years.