Mulcair is to visit Winnipeg today, just as floodwaters are expected to crest in the southwestern part of the province.
With the help of Premier Greg Selinger's office, Mulcair had arranged to get a briefing and a tour of a military command post that's co-ordinating the work of 500 soldiers who've been helping fill sandbags and shore up dikes along the swollen Assiniboine River.
Brig. Gen. Christian Juneau, commander of the third Canadian Division, had agreed to the tour, according to Mulcair's office. All that was needed was a green light from Defence Minister Rob Nicholson.
Late Tuesday, Mulcair was informed that Nicholson had vetoed the visit.
"It's disheartening to see the Conservatives play politics during a crisis," said Mulcair spokesman George Smith.
"This was a simple briefing that had been arranged with the military. There was no reason for the minister to go out of his way and intervene."
A spokeswoman for Nicholson said the government has nixed all tours for the time being.
"The next 48 hours are key to dealing with the Manitoba flood as it is expected to push through Portage La Prairie," Johanna Quinney said in an email.
"As a result, no tours will be approved at this time so our men and women on the ground can focus all their efforts (on) keeping Manitobans and their properties safe during this crucial time."
There were no similar qualms about disrupting crucial efforts to prepare for the crest on Sunday, when the prime minister took a 20-minute helicopter tour of the flood zone, accompanied by Selinger and local MPs.
"Obviously, we are here to express our solidarity with people, as I know everybody is very concerned about the situation," Harper said after a briefing with emergency personnel at Brandon City Hall.
Selinger's NDP government has welcomed politicians of all partisan stripes who want to see the extent of the crisis for themselves. But the premier refused Wednesday to get dragged into a partisan scuffle over Mulcair's cancelled tour.
"The federal government makes its own decisions about who they can allow to be here," Selinger said during a media briefing.
"We think everybody needs to understand the gravity of the situation but ... we're focusing on making sure we're in a position to make the best decisions as rapidly as possible to protect people and property."
Selinger declared a state of emergency last Friday and asked for military assistance to help communities at risk of being submerged to prepare for the floodwater crest.
This summer's flood, triggered by torrential rain and water pouring in from Saskatchewan, could break records set during one of Manitoba's worst floods in 2011.