From Politics 101, a few basic principles. Elections are about incumbents more than they are about challengers. Incumbents usually have a fairly definite corps of supporters. Challengers, as a total, therefore have a limited pool to draw from; the more challengers deeply divide the anti-incumbent vote, the more likely the incumbent is to win. A weak incumbent may lose to a single challenger; two reasonable strong challengers in the field greatly improves the incumbent's odds (excepting in cases of runoffs).
That has relevance in the case of the Oregon 1st district, where incumbent Democrat David Wu is facing a strong challenge from state Labor Commissioner Brad Avakian, who announced last week. (One indicator: Facebook page is up to 845.) If those two are in the Democratic primary, Wu may lose to Avakian. If more strong candidates appear, Wu may prevail.
It's a basic calculus that probably has been on the minds of quite a few Democrats - and Republicans, since a weakened Democratic nominee could open the seat for a Republican candidate.
The point is spelled out with more detail in a Jon Isaacs piece on Blue Oregon, noting as well that several other Democrats are also considering entering the 1st district race. If you see fast moves by Avakian in the next couple of weeks, toward gathering endorsements and support, bear this in mind: His chances may depend heavily o whether any of those other prospects in fact jump in.