The federal government can and will still enforce its more severe marijuana laws against residents of states without such laws.
The Supreme Court has taken this case (Gonzales v. Raich) and decided that state medical marijuana laws provide no protection. The court decided 6-3 against medical marijuana laws with one justice a cancer patient and one's husband a cancer patient at the time providing two surprise votes for sanity; most observers expect such state reforms to never again have more than one vote (Clarence Thomas) on the court since those two soon died and retired.
Obama has been an exceptionally lenient president, reducing the prosecution of medical marijuana patients so long as they aren't caught in his frequent raids of buyers' clubs and aren't involved in any kind of commerce or production. Even licensed facilities have been granted no leniency. States that consider state provision and regulation of marijuana have been consistently threatened by Obama's AG Eric Holder with prosecution and incarceration of state officials.
That will likely be enough to defeat legalization initiatives and certainly end the likelihood of measures passed through the legislatures.
But if legalization initiatives pass, expect a new growing industry of federal prosecutions to reverse the new laws. When the Republicans next elect a president, the states that tolerate marijuana will probably see massive punishment doled out until the laws change back.
Actual legal marijuana is not likely to be seen anywhere in America before 2030 and then only if the pace of reform continues to accelerate.
Changes in public opinion are really insufficient in the near term. This is politics. Fear, inertia, and moneyed interestes are arrayed strongly against legalization. The voice of the people enters the discussion weakly and slowly, but it can win over a long period of time.