WASHINGTON, DC-JUNE 11: Congressman Steve Daines (R-MTO) at a dinner party in honor of new Japanese Ambassador to the U.S., Kenichiro Sasae and his wife, Nobuko. The evening was hosted by the captivating Roy Pfautch, long-time friend of the new Ambassador and international consulatant, and was held at the Willard Hotel Tuesday June 11, 2013 in Washington DC. (Photo by Rebecca D'Angelo For the Washington Post) Rep. Steve Daines (R-Mont.)  (Rebecca D'Angelo for The Washington Post)

Freshman Rep. Steve Daines (R) will enter the open Montana Senate race Wednesday, GOP aides said Tuesday.

Rob Collins, executive director of the National Republican Senatorial Committee, confirmed that Daines will run, as was widely expected. Daines previously scheduled a "special event" for Wednesday in Bozeman, Mont., and has been staffing up for a run at the upper chamber.

"Montana, tomorrow, Steve Daines is going to get in the race, and he's an impressive guy," Collins told reporters Tuesday morning. "In a year where we lost the governor's race and the Senate race (in 2012), he won by a healthy margin."

Another GOP aide close to Montana politics confirmed that Daines was set to enter the race Wednesday.

Collins added of Daines: "He's running in a state that's tough for (President) Obama, but we understand that the Democrats have held that seat for 100 years. So we have a challenge."

Collins noted that Montana Lt. Gov. John Walsh was not the Democrats' first choice to run for the open seat — that was former governor Brian Schweitzer — and Republicans were confident he was someone they could define.

Rep. Denny Rehberg (R) lost to Sen. Jon Tester (D-Mont.) last year, and the GOP lost a disappointing race in the governor's contest as well — a race that put Walsh into elective office for the first time.

Daines is a relative newcomer too, having won his first office in 2012 when Rehberg retired. He was the GOP's lieutenant governor nominee on a 2008 ticket that lost badly to Schweitzer.

The Democratic Senatorial Campaign Committee signaled it will attack Daines as a "Washington insider."

"Montanans will reject Steve Daines’ reckless and irresponsible political agenda and support a hero like John Walsh, whose courageous service and deep Montana roots make him the best candidate to represent Montana values in the US Senate," said DSCC spokesman Justin Barasky.

Sen. Max Baucus (D-Mont.) is retiring.

Updated at 4:12 p.m. with DSCC comment.