Will and Brooke are discussing national politics, and Will is trying to make Brooke understand that he can't claim to agree with everyone. He needs to take a stance – rather than say that he supports Reform in a general way, he should say that he plans to support the old leaders who represent the status quo.
Brooke never seems to get it (he's not that bright).
Overall, Will is pleased with his job at the Pioneer (which includes editing newspaper articles to persuade Middlemarchers that Reform is a good idea, and also includes educating his employer, Mr. Brooke, about the right way to enter politics).
By working for the newspaper, Will seems to belong to no social rank at all – and for English society in the nineteenth century, social rank was everything. Voluntarily giving up a high social rank to work for a newspaper seems like a crazy choice to most people around him.
The fact that Will belongs to no social rank, combined with the fact that he's part Polish, makes many Middlemarchers wary of him. People in Middlemarch like order, stability, and tradition. They like things to be cut and dried. And Will is anything but – they don't know what to make of him and think he might be vaguely dangerous.
The conservative newspaper, the Trumpet, takes advantage of this, and prints negative stories about Will, hinting that he's a revolutionary and a foreigner, sent from Poland to stir up discontent in England.
Historical context note! This story, as you know, takes place in 1830, only 30 to 40 years after the bloody French Revolution broke out and not long after the final defeat of the French Emperor Napoleon. Around this time, there were revolutions all over Europe, and the English were afraid that the Revolutionary fever would be contagious, and would spread to England. So, although the idea that Will is a foreigner sent to stir up revolutionary spirit in England seems laughable, it was a common fear at the time. Conservatives didn't trust foreigners.
And back to the story.
Will's not a revolutionary, but he does enjoy the feeling of having no social class. He likes to "slum" by hanging out with poor children, who follow him around like a band of puppies.
Not everyone minds his "eccentricity" (which is what conservative Victorian English people called anything that seemed at all out of the ordinary), and he makes friends with several important families.
He visits the Bulstrodes, because they're liberal, too, but he's not really friends with them (they're too uptight, and don't like for him to lie down on the rug in the living room).
He has become friends with the Farebrothers and with the Lydgates, especially.
The scene shifts to the Lydgates' house, where Will and Mr. Lydgate are discussing politics while Rosamond gets the tea.
The take-home message from their discussion is that Lydgate doesn't think Brooke is fit to be in Parliament, but Will disagrees. He says that Brooke might not be the smartest, but at least will support the Reform bill, which is all that matters.
Lydgate thinks Will is oversimplifying things.
The argument continues into a question of motives and corruption. Both of them think that the other is insinuating something, and both of them get defensive.
Lydgate's still worried that people will think that he expects some kind of personal pay-off from Bulstrode for supporting the new hospital (and for voting against Farebrother in the election of the chaplain), and Will is afraid that people will suspect him of some kind of personal, ulterior motive in his dealings with Mr. Brooke.
After Will leaves, Rosamond asks Lydgate why he was in a bad mood. He doesn't usually argue with Will, even when Will is moody.
Lydgate tells her it was nothing, but really it was that he'd gotten a bill for the new furniture.
He doesn't want to tell her because she's pregnant, and he doesn't think she should be upset.