It is no longer out of the question that the Democrats could gain a filibuster-proof majority in the Senate -- they need to net 9 seats, and GOP-held seats are in jeopardy in Virginia, New Hampshire, New Mexico, Colorado, Mississippi, Minnesota, Oregon, Kentucky, North Carolina, Alaska and possibly Texas and Maine. This does not mean the GOP will lose all of these seats, but if the Democrats win most of the close races, they will have a shot at 60. The GOP has a shot at picking up Louisiana.
The Libertarian Barr-Root ticket is the worst possible combination for McCain. They could pick off enough votes in Nevada and Colorado to give both states to Obama.
New Mexico was a tossup leaning to Obama, but now is even more at risk. Nevada was leaning to McCain, and that may be Root's strongest state. In the end third party candidates tend to fade in close elections, but this year McCain will keep tacking left to win votes of Democrats and independents uncomfortable with Obama, and this will leave more on the right open for Barr and Root.
In order of risk: Barr-Root will do well in Nevada, Colorado and New Mexico -- all tossup states. They could make Georgia, Montana and Alaska tighter, but probably not tip these states to Obama. I don't see much impact in Midwestern swing states.