A disaster in every respect.
For him to have even a slight chance in hell of passing a lot of his agenda he would need a sympathetic Congress. Something that, even with the great liberal reawakening, just would be damn near impossible just two years after the 1894 Congressional Elections. Sure, he could possibly get something by the Senate, but the likely pro-Gold Republican House majority would probably obstruct him at every turn. They did, after all, just spend a campaign season painting him as a radical.
Likely, he ends up becoming the next John Tyler and is either defeated at the Democratic Convention of 1900 or is destroyed by the GOP candidate in 1900. The Interests in the country would not let a man like Bryan defy fate for a second time.
For a Bryan presidency, or a victory for that matter, to be successful would pretty much require William J. Bryan to tone down his rhetoric and even give a few concessions to Tammany Hall on things like alcohol and immigration. Something that, of course, the real Bryan would never do and why William McKinley was able to capitalize with ethnic Democrats in a way that no Republican before him could. Any Democrat who had a dream of winning a presidential election, pre-1936, couldn't afford to overly offend the urban machines which was one of Bryan's defining characteristics and why the Democrats spent nearly a generation out of the White House.
NOTE: edited the post given that the Democratic Presidential nominees of the late 19th century were mostly "reform" Democrats who got popular because of their opposition to machine "corruption". Of course, it is really dubious if they were actually opposed to the corruption of the machines or just didn't like the idea of the "uncultured" having a say in how the Democratic Party was ran. Point isn't really that any opposition to Tammany was horrible, but the kind of impassioned opposition of Bryan and his ilk had against the group was on a level more damaging than the kind of opposition Cleveland had. After all, at least Cleveland was a wet who didn't give the image of being suspicious of new arrivals. Bryan seemed to realize some of his mistakes with reaching out to urban ethnic voters in 1900, when he campaigned extensively on anti-imperialism. Also, courtesy cartoon!
