While a series of marches focused much of the nation's attention on the plight of illegal immigrants, scores of other Americans quietly seethed. Now, with the same full-throated cry expressed by those in the country illegally, they are shouting back. Congressional leaders in Washington have gotten bricks in the mail from a group that advocates building a border fence, states in the West and South have drawn up tough anti-immigrant laws, and ordinary citizens, such as Janis McDonald of Pennsylvania, who considers herself a liberal, are not mincing words in expressing their displeasure.