<p>Where is this sentence from? It's a pretty vague and bad sentence. Because "one" is the actual word the verb number should depend on, since "of congressmen" is prepositional.</p>
<p>I guess with "no error" the sentence is supposed be read in this way: </p>
<p>senator frank as one [among those] congressmen who are going to attend...</p>
<p>As opposed to:</p>
<p>senator frank as one who is going to attend.</p>
<p>Yes, "of congressmen" is prespositional, but included in the prepositional phrase is "who are going to attend the conference" -- the "are" refers to "Congressmen" and not to "one." Kind of crazy!</p>