Depends on the GOP candidate. In the past it's seemed that a moderate Democrate has been prefereable to a conservative Republican. Marinovich is a moderate, pro-business Democrat who can honestly say she has lowered taxes. Add to that the fact that the 3rd district includes Wyandotte and Lawrence and you've got the two Democratic strongholds in the state giving their candidate a leg up.
Who is the "moderate Democrat" ? Dennis Moore is a down-the-line liberal who scored a 16% ACU rating in 2005 in a clearly GOP district. If the RINOs hadn't knifed the distinguished Vince Snowbarger, he'd still be the Congressman today. They're going to try to knife the equally distinguished Conservative Attorney-General Phill Kline this November with one of their apostates running as a 'Rat.
But if a Kansas City Democrat runs, the GOP candidate will clean up in Johnson County and win the election. Dennis Moore hasn't won 5 times by getting slammed in Johnson County; he's always gotten at least 45% of the vote there.
You're correct, of course, that including all of Wyandotte County and most of Douglas County in the district has kept Moore alive all these years. Your beloved RINOs proved once again how they're hell-bent on handing power over to the Democrats when they refused to adopt a redistricting plan that would lead to Moore's defeat and instead gave him a less Republican district than before. It would have been quite easy to draw 4 districts that each gave President Bush at least 57% in 2000 (he got 59% in the state as a whole), but Governor Graves and the rest of the RINOs were not interested.