US President Donald Trump has taken to Twitter to attack Republican Mitt Romney over a critique from the former presidential nominee.

Incoming Senator Romney issued harsh criticism of Mr Trump, saying he “has not risen to the mantle of the office.”

Mr Trump didn’t take too kindly to the comments though and called on Mr Romney to be a team player.

The barbed remarks from the 2012 Republican presidential candidate two days before he and other politicians take up their seats in the new Congress seemed to suggest Sen. Romney is positioning himself to take over the role of Jeff Flake, a departing senator who was a vocal critic of Mr Trump.

US media said Sen. Romney’s comments even prompted suggestions online that he sounds like he is considering challenging Mr Trump for the nomination in 2020.

“With the nation so divided, resentful and angry, presidential leadership in qualities of character is indispensable,” Sen. Romney wrote in an op-ed piece in The Washington Post.

“And it is in this province where the incumbent’s shortfall has been most glaring.”

It is the latest twist in a relationship that has run hot and cold for years.

And it comes at a tough time for Trump as he begins to ‘feel the heat’ over his US government shutdown.

Top congressional Democrats are calling on President Donald Trump to back bills that would re-open the federal government, declaring that he was using the American people as “hostages” in the fight for a border wall.

House Democratic leader Nancy Pelosi and Senate Democratic leader Chuck Schumer met Trump at the White House overnight for a briefing on border security. The session did not yield any breakthroughs, and Democrats said they remained committed to introducing the legislation later today.

The administration has so far rejected the plan, which does not include funding to build a wall along the US-Mexico border.

President Trump says the border wall was a key election promise, though he had insisted Mexico would be made to pay for it.

Schumer said Trump could not provide a “good answer” for opposing the new spending bills. He added that Trump and Republicans “are now feeling the heat.”

Sen. Romney called presidential candidate Mr Trump a fraud and a phony in 2016. But last year he thanked Mr Trump for his endorsement as he ran for a senate seat in Utah.

In the op-ed piece, Sen. Romney noted he had not supported Mr Trump as the Republican Party’s nominee in 2016.

He said he had hoped the Trump campaign would refrain from name-calling and had not. He said he was initially encouraged when Mr Trump brought into his cabinet steady-hand figures like General James Mattis as Defence Secretary and John Kelly as chief of staff.

“But, on balance, his conduct over the past two years, particularly his actions this month, is evidence that the president has not risen to the mantle of the office,” Sen. Romney wrote.

Sen. Romney said last month — when Mr Trump abruptly announced he was pulling US troops out of Syria, and Gen. Mattis resigned in protest — was particularly disturbing.

“The Trump presidency made a deep descent in December,” he wrote.

Sen. Romney said he will treat Mr Trump as he would any president, either in or outside his party.

“I do not intend to comment on every tweet or fault,” he wrote. “But I will speak out against significant statements or actions that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions.”