My understanding is the milder climates in Europe are caused by there being an ocean to the west of it - the western United States gets the same sort of temperatures.

Very little of the western US has temperatures as mild as are typical in Europe. Once you go even a few miles inland, extreme temperatures become common. For example, the Inland Empire suburbs of LA are notoriously hot in summer, and even temperatures in the 100s (Fahrenheit) are normal during summer.

As to the walkable cities - that's actually General Motors' fault.

You may want to actually read that wikipedia link, since it doesn't actually support the claim you're making. It points out that the streetcar systems that were dismantled were mostly hemorrhaging money and even in cities GM did not touch, the streetcar systems were usually dismantled anyway.

If climate also contributes, then I'd expect west coast American cities to be more walkable than east coast and inland cities.

They are more walkable ceteris parabis. The east coast is more densely populated, which puts more pressure on people to use public transit. But SF, Seattle, Portland, and Oakland are among America's most walkable cities.

Western cities probably also tend to favor walking because people there are more liberal, and so would want walkable and bikable cities partly for political reasons.

The West Coast, East Coast, and Northern Midwest are all liberal. Also keep in mind that liberals probably gravitate toward walkable cities because it fits with the lifestyle they enjoy. Conservatives are more home-and-family oriented than liberals, so they especially like having big homes and big cars to facilitate family togetherness.