DENVER -- Democratic Congressman Mark Udall will officially launch his Senate campaign Tuesday.

He is expected to tour 16 Colorado communities over four days.

Tuesday's schedule includes stops in Colorado Springs at 7:45 a.m., Gunnison at 10:15 a.m., Grand Junction at 12:30 p.m., Craig at 3 p.m. and Denver at 5:30 p.m.

Former Congressman Bob Schaffer is seeking the Republican nomination for the seat being vacated by retiring Sen. Wayne Allard.

Schaffer is expected to accept his party's nomination Saturday at the Republican's Colorado convention.

According to published reports, Schaffer's campaign manager, Dick Wadhams, expects an official kickoff will happen shortly thereafter.

Three other people have expressed interest in the seat; Democrat Mark Benner, Green Party candidate Bob Kinsey and Independent Buddy Moore.

Learn more about each of the candidates:

Mark Brenner's campaign Web site.

Bob Kinsey's campaign Web site.

Buddy Moore's campaign Web site.

Bob Schaffer's campaign Web site.

Mark Udall's campaign Web site.