The Liberals and NDP have been eyeing the riding, which has historically been Tory territory, as observers on Parliament Hill suggest it could be up for grabs in the Nov. 25 byelection.
"I've been working very hard, as has our candidate, Rolf Dinsdale, and we're extremely optimistic about the kind of response that people have given there on the ground," Liberal Leader Justin Trudeau said Wednesday.
The opposition parties are hoping the Senate expense scandal and other controversies in Ottawa will prompt voters not to vote Conservative this time around.
Treasury Board President Tony Clement would not comment on whether the Conservatives can hang on to the riding.
"You guys are always very good about predicting results before they happen," he told reporters.
"Let's just wait for the people of Brandon-Souris to make their decision."
Trudeau and NDP Leader Tom Mulcair have visited Brandon-Souris more than once in recent weeks, and Green Party Leader Elizabeth May has also stopped in the riding.
The race for the seat has become so heated, attack ads targeting the Liberals have been distributed in the riding.
Kelly Saunders, an associate professor in politics at Brandon University, says attack ads are usually only seen at a federal level, and their distribution shows the stakes for the byelection are quite high.
And in a move that political watchers say is unusual for byelection campaigns, Brandon residents have started receiving a letter from Prime Minister Stephen Harper, criticizing the Liberals.
CBC News has confirmed that the letter came from Harper.
'This is the riding to watch'
Whoever wins in Brandon-Souris will replace former Conservative MP Merv Tweed, who resigned in August to take a top job at rail company OmniTrax.
"This is the riding to watch," Saunders said.
"It's pretty exciting because I think this is really going to set the stage for 2015. As all the parties start moving towards that general federal election, I think it's going to be telling about how the parties are going to be position themselves."
Saunders said she believes the byelection will be a fight for "every single vote."
In Brandon-Souris, a total of 2,422 voters took part in advance polls ahead of the Nov. 25 byelection, according to preliminary estimates released by Elections Canada on Tuesday.
That's only several hundred votes less than the 2,754 voters from the riding who cast ballots in advance polls in the last federal election.
The numbers surprise Saunders, who said voter turnout tends to be much lower in byelections because they are generally not considered to be political game-changers.
"This seems to be a little bit of a different story," she said.
Candidates faced off at debate
The Liberal candidate in Brandon-Souris is Rolf Dinsdale, whose father, Walter Dinsdale, was the area's Progressive Conservative member of Parliament for 32 years.
Running for the Conservatives is former Manitoba MLA Larry Maguire, who has said the recent poll numbers don't faze him.
Representing the NDP is Cory Szczepanski, a welder and a former president of the Brandon District Labour Council.
Also running in the byelection are David Neufeld of the Green Party and Frank Godon of the Libertarian Party.
On Wednesday, all five candidates faced off in a lunch-hour debate — the third of four being held in the riding — in Brandon, hosted by the local chamber of commerce.
Maguire had not attended the previous two debates, including one held Monday at Brandon University.
Following Wednesday's debate, Maguire answered no questions from the media and he dodged a scheduled interview with a CBC News reporter.
A Conservative spokesperson said Maguire's schedule had changed. His office has not rescheduled the interview as of Wednesday evening.
The fourth all-candidate debate will take place Thursday evening at the Keystone Centre.
The other federal byelection taking place in Manitoba on Nov. 25 is in Provencher, which has been without an MP since former cabinet minister Vic Toews resigned in July.
Elections Canada says 2,313 people in Provencher voted in byelection advance polls. Leading up to the last general election, 3,218 voters took part in advance polls there.