A popular misperception is that the incumbent is the biggest problem for a challenger. It's simply not true. The first task for a challenger is to get the voters to know who you are. The incumbent generally does not matter for this task. At some point you may need to show why you're better than the incumbent, but if the voters don't even know who you are, you're not going to get there.
Consider a NY congressional district. There are roughly 660,000 people in each district, with 450,000 registered voters. How do you let these people know who you are? Sending each voter one postcard would cost about $100,000, and one postcard is not going to do the job.
Direct mail may not be the most cost-effective means. My district (NY-21) is entirely in one moderate media market. A 30-second spot on the 6:00 news costs about $500. $50K would buy a significant TV ad campaign. Not enough, but it would be noticed. Doing a solid combined package of TV and radio for $100K would probably be more effective than the postcard. In 2004 I spent only $25K on TV and radio and did 5 points better than the last 7 challengers in the district.
Other districts are more difficult. NY-20, home to a hot race this year, is in 4 media markets, including the New York City market. A challenger in that district would have to spend at least 4 times as much on TV and radio to get the same effect. A well-run PR campaign of press releases and events might get substantial media coverage, but it will still be more expensive to have PR professionals reach media in 4 markets. The postcard approach might be more cost-effective in such a district.
Challengers face many other hurdles. Few understand the various techniques that strengthen a campaign, such as the value of a simple message, repeated with frequency. Raising money for challengers is also extremely difficult.
Political Science, especially the work of Gary Jacobson (now at UCSD), has shown that challenger spending is dramatically more important than incumbent spending in determining the outcome of elections. Yet the overwhelming majority of contributions go to incumbents. Parties and PACs focus on protecting incumbents, while the media gives them free press.
As long as these circumstances prevail, we will not live in a genuine democracy. Solutions might include offering a substantial amount of free postage for candidates (enough for, say, three postcards per registered voter), or the creation of PACs that contribute only to challengers.