How did the Oligarchy try to prevent Ernest’s election to government? Were they successful?

The Oligarchy, also known as the Iron Heel, did not prevent Ernest from being elected to Congress in the fall of 1912 by crushing the middle class economically. He was still elected, as were many other socialist candidates.
Ernest ran for Congress in 1912, nominated by the socialist party of California. At first, everyone believed he had a good chance to win. He had a few events in his favor:
The San Francisco street car strike was broken.
The teamsters' strike was broken.
The violence with which the strikes were broken caused the unions to throw their influence behind Ernest's campaign. Groups like the Water Front Foundation also supported the teamsters and, after the strike, began to support Ernest and the socialists. Groups involved in or supporting the strike turned from the labor party and began to support the socialist party and, by extension, Ernest's run for Congress.
London writes:

The whole Water Front Federation, along with its allies in the structural trades, had backed up the teamsters, and all had smashed down ingloriously. It had been a bloody strike. The police had broken countless heads with their riot clubs; and the death list had been augmented by the turning loose of a machine-gun on the strikers from the barns of the Marsden Special Delivery Company.

The violence used against the workers was enough to incite the unions to change their political course. Their support made it more likely that Ernest would win. The increase in the number of people supporting the socialist party also invigorated the party. Even though that support came from a bad place—the mistreatment of the workers—it began to create what looked to be a change in national leadership. London says:

Beaten on their chosen field, they were ripe to seek revenge by means of political action. They still maintained their labor organization, and this gave them strength in the political struggle that was on. Ernest's chance for election grew stronger and stronger. Day by day unions and more unions voted their support to the socialists, until even Ernest laughed when the Undertakers' Assistants and the Chicken Pickers fell into line. Labor became mulish. While it packed the socialist meetings with mad enthusiasm, it was impervious to the wiles of the old-party politicians. The old-party orators were usually greeted with empty halls, though occasionally they encountered full halls where they were so roughly handled that more than once it was necessary to call out the police reserves.  

Unfortunately, the Iron Heel did not want to let socialists take any real power in Congress. The people who controlled the Oligarchy were mostly wealthy business owners and financiers; they were people who owned banks or worked on Wall Street. As they saw laborers and unions turning to support the socialist party, they forced businesses into bankruptcy and secured their own power with their land-rights and mercenaries. As London says, "Thus, the summer of 1912 witnessed the virtual death-thrust to the middle class." 
Watching this changes Ernest. Though he once believed that political change could be wrought from elections and peaceful democracy, he doesn't believe it once the middle class is broken. Even before the election occurs, he knows that winning won't change anything. His reaction is one of a transformed man. Ernest says:

"It's no use. . . . We are beaten. The Iron Heel is here. I had hoped for a peaceable victory at the ballot-box. I was wrong. Wickson was right. We shall be robbed of our few remaining liberties; the Iron Heel will walk upon our faces; nothing remains but a bloody revolution of the working class. Of course we will win, but I shudder to think of it."

London goes on to say that Ernest "pinned his faith in revolution after that." The socialist party didn't come to agree with him for some time; his fellows assure him that he'll win a Congressional seat. He asks what will happen when the Oligarchy murders him. That time, they assure him, will be the time to rise up.
Ernest wins the election, which doesn't surprise him. The Democratic party is essentially destroyed and so voters could only choose between the Republican party and the socialists. People who would have voted for the Democratic candidate align with the socialists. Despite the decimation of the middle class, Ernest still gets their votes and is elected to Congress. As London writes, "Of course Ernest was elected to Congress in the great socialist landslide that took place in the fall of 1912."
While many of his socialist friends are excited by his victory and the other Congressional seats they've won, Ernest remains skeptical that change can happen without revolution, even after his election. The socialists send 50 elected officials to Congress but still have a minority in Congress. Ultimately, of course, Ernest is right. The Iron Heel takes control of the United States and maintains it for a very long time after Ernest is ultimately executed.