What is the difference between Evangelical and Protestant? Just the other day, I heard somebody talking politics and contrasted the Roman Catholic church's political influence with "the Evangelical vote". In this sense, as is often done in US media, the word seem to be used as a synonym of 'Protestant'.

However I get the feeling there is another layer to this. Where did the term 'Evangelical' come from and how is it distinct from 'Protestant'. Used properly, what group(s) is it generally understood to include or what world view does it describe?
"Evangelical" is certainly difficult to pin down! The Encyclopedia of Christianity says


  [In America] it covers a wide range of not completely harmonious uses, from the Pentecostal churches to the peace churches, Missouri Synod Lutherans, Southern Baptist Convention, Holiness movement (eg Church of the Nazarene), charismatic groups (including Roman Catholics), and evangelicals in the mainline denominations. 1


and there seems to be a flourishing cottage industry of evangelicals writing books and articles worrying about how to define themselves.

History

John Stott emphasized that evangelicalism does not see itself as an innovation or deviation from orthodox Christianity. Doctrinally speaking, evangelicals are able to find many precursors - for example, Stott identifies Augustine as a "proto-evangelical", on the basis of his view of grace. 2

The term "evangelical", which derives from the Greek evangelion (gospel, good news), does not seem to have been used to describe a distinct group within the church until roughly the Reformation era. Martin Luther picked it up, and today the German word Evangelisch is not really distinguishable from Protestantisch - unlike in the English-speaking world.

Stott and others have identified several related movements that have identified themselves as evangelical. The following listing is from John Hitchen, who argues that evangelicalism has tended to define itself in opposition to tendencies in the church, or society, at large.


  
  The Reformers - against medieval Catholicism.
  The First Evangelical awakening - against the 'deadness' of the contemporary church.
  The Second Evangelical awakening - against the Deism, laxity and Unitarianism in the early 18th century church.
  The Clapham Sect and early 19th Century evangelicals - against lack of 'seriousness' and nominalism in the established church(es).
  The 'Fundamentalists' of the early 20th century - against proponents of the Social Gospel and Higher Criticism.
  The mid 20th century evangelicalism - against Liberal theology and Neo-orthodoxy.3 
  


Other modern listings might include subgroups emphasizing social justice, the gifts of the Spirit, or ecumenism, although the boundaries here may not be very sharp 2. 

Defining attitudes

In all of these cases, the groups involved did share some common doctrines or positions - in particular, they were trying to preserve or restore 'essential' or 'original' Christian practice and belief. They also share a desire for conversion of the world - in this regard, Stott contrasts it on either side with the fundamentalist position (stand aloof from the world) and the liberal position (adapt to the world) 2. 

David Bebbington identified four ideas that form a 'common core', or 'quadrilateral of priorities':


  conversionism, the belief that lives need to be changed;
  activism, the expression of the gospel in effort;
  biblicism, a particular regard for the Bible; and what may be called
  crucicentrism, a stress on the sacrifice of Christ on the cross.4


This is a more 'sociological' classification of evangelicals, in terms of their prevailing attitudes rather than any particular point of doctrine. Evangelicals may be strict about doctrine, but the actual content of doctrine isn't tremendously distinctive from 'ordinary' Protestant beliefs, with the exceptions of (1) a stronger view of Biblical authority than many others have, and (2) a related commitment to 'pure' or 'mere' Christianity 5. The Bebbington scheme has gained some currency, though Stott for one was uncomfortable with all the -isms, and the low profile of God. He preferred this formulation of evangelical priorities:


  The revealing initiative of God the Father, the redeeming work of God the Son, and the transforming ministry of God the Holy Spirit. 2


Politics

In terms of American politics, I don't think it's straightforward to predict where evangelicals will end up. Moreover, the media tend to use the classification fairly carelessly, as a synonym for 'conservative Christian' or 'Republican churchgoer'. In fact, I think there is a great diversity of political beliefs among evangelicals (though the two-party system suppresses that diversity at the polls). Part of this is generational or age-based, but I also have a sense that some political points spring fairly naturally from the evangelical worldview.

For example, the evangelical environmental movement makes perfect sense within the context of the history above. It's a matter of engaging with the world in a positive way, in order to take care of it (and it can be seen as oppositional, like in the Hitchen list above). Equally, many evangelicals care about social and economic injustice in a manner that doesn't match the stereotype that evangelicals ipso facto have right-wing views.
This is not to say that evangelicals are automatically left-wing or liberal either! Probably, most evangelicals would be considered socially conservative, and may even regard such issues as overriding priorities when choosing candidates. For what it's worth, I don't think that an evangelical's 'ideal' US political party would greatly resemble either the Republicans or the Democrats, but they do seem to be the only games in town. 

1. Encyclopedia of Christianity (Wm. Eerdmans, 2001) under "Evangelical Movement"
2. Evangelical truth: a personal plea for unity, integrity and faithfulness, John Stott (InterVarsity Press, 2005)
3. What it means to be an evangelical today, John M. Hitchen, Evangelical Quarterly 76(1):47-64, 2004.
4. Evangelicalism in modern Britain: a history from the 1730s to the 1980s, D. W. Bebbington (Unwin Hyman, 1989).
5. The Evangelical Anglican Identity Problem, J. L. Packer (Latimer House, 1978).