Of course that's true, but what's faulty is your assumption that Republican federal candidates will do better on a Congressional district level than they'd do on the state level.
That assumption no longer really holds any water following last November's elections -- on the contrary, it increasingly seems that GOP strength is concentrated in a relatively small number of heavily conservative districts. The marginal districts in many places (classic example: C0-07) are, as of late, trending pretty Democratic and I suspect that that will be true in Presidential races as well.
As soon as that situation is resolved (one way or another), the loony left is going to lose all the anti-war supporters who are otherwise fairly moderate. They aren’t going to support the rest of the radicalist agenda that the DC dems are moving to. The old rally cries aren’t what they used to be; the race card is tattered by overplay, the feminist “equality” agenda is accomplished leaving only the “womyn”, the homosexuals are only 3-4%ers, and unions continue to decline.
The margins they gained in ‘06 were razor thin and built on deception by new Dems representing themselves as far more conservative than their leadership has let them vote, and on the apathy of the Republican base. Both of those situations can be turned quickly, if targeted by the RNC and the individual GOP candidates. All they need do is break themselves of the habit of “support the incumbent” thinking, and re-target the money and support to candidates with the resonant messages and fresh thinking.