Name any elected "liberal", D or R, who would ever consider legalizing MJ. The only elected official that I can think of are outgoing Republican Governor Gary Johnson of NM and TX Republican Rep. Ron Paul): no "liberals", they.
The WOD is the most "liberal" (i.e. leftist) government program of all, as it seeks to impose total government control of what people put into their bodies, and prescribes confinement in anal-rape camps for any non-connected person who violates said diktak. Legalization (or, more accurately, re-legalization) is a truly conservative issue.
Second, if we make the assumption that most Republicans are conservatives and most Democrats are liberals, you're saying that legalization would be supported mainly by Republicans. I beg to differ.
H.R. 2592, States' Rights to Medical Marijuana Act, calls for the amending of federal law so that states that wish to permit the legal use of marijuana as a medicine for seriously ill patients may do so, without interference from the federal government. Sounds like a good conservative issue, doesn't it? Let's look at the bill's co-sponsors:
California
Mike Thompson (D 1st)
Lynn Woolsey (D 6th)
George Miller (D 7th)
Nancy Pelosi (D 8th)
Barbara Lee (D 9th)
Tom Lantos (D 12th)
Fortney Stark (D 13th)
Zoe Lofgren (D 16th)
Sam Farr (D 17th)
Brad Sherman (D 24th)
Henry Waxman (D 29th)
Maxine Waters (D 35th)
Dana Rohrabacher (R 45th)
Bob Filner (D 50th)
Colorado
Mark Udall (D 2nd)
Hawaii
Patsy Mink (D 2nd)
Illinois
Janice Schakowsky (D 9th)
Indiana
Julia Carson (D 10th)
Massachusetts
John Olver (D 1st)
James McGovern (D 3rd)
Barney Frank (D 4th)
Michael Capuano (D 8th)
Maryland
Robert Ehrlich (R 2nd)
Albert Wynn (D 4th)
David Bonior (D 10th)
Lynn Rivers (D 13th)
Michigan
David Bonior (D 10th)
Lynn Rivers (D 13th)
John Conyers (D 14th)
Carolyn Kilpatrick (D 15th)
Minnesota
Martin Olav Sabo (D 5th)
New Jersey
Robert Andrews (D 1st)
New York
Jerrold Nadler (D 8th)
Maurice Hinchey (D 26th)
Oregon
Earl Blumenauer (D 3rd)
Texas
Ron Paul (R 14th)
Vermont
Bernard Sanders (I At-Large)
Washington
Jim McDermott (D 7th)
Wisconsin
Tammy Baldwin (D 2nd)
I count 35 Democrats, 3 Republicans, and 1 Independent. You may want to re-think your theory.