President Donald Trump's attack on Senate Majority Leader Mitch McConnell was the "single dumbest thing Donald Trump's done in office," Sara Fagen, a former senior aide to President George W. Bush told CNBC on Monday.
Last week, Trump slammed McConnell for the lack of progress on the Republican agenda.
"Mitch, get back to work and put Repeal & Replace, Tax Reform & Cuts and a great Infrastructure Bill on my desk for signing," Trump wrote Thursday on Twitter.
Fagen, also a CNBC contributor, believes that was a tactical error.
"He needs the Senate majority leader to get his tax reform package through," the former political director for Bush said in an interview with "Power Lunch."
In fact, she said McConnell is the "single most important person" in passing Trump's agenda.
After the taunts, Republican senators rallied around McConnell.
Sen. John Cornyn, R-Texas, the second-ranking Senate Republican, on Friday said, "No one is more qualified" to head the Senate in the "team effort" to pass Trump's agenda.
The White House did not immediately respond to a request for comment.
— CNBC's Jacob Pramuk contributed to this report.
WATCH: GOP senators back Senate Majority Leader McConnell after Trump's taunts
