"White Republican evangelical Protestants express the greatest willingness to consider voting for a conservative third-party candidate. More than half of white Republican evangelicals (55%) say they would consider this, compared with 36% of white Republican mainline Protestants and the same proportion of white Republican Catholics."
The entire, lengthy Pew Report is worth a read. From the intro:
year before the 2008 presidential election, most major national opinion trends decidedly favor the Democrats. Discontent with the state of the nation is markedly greater than it was four years ago. President Bush's approval rating has fallen from 50% to 30% over this period. And the Democrats' advantage over the Republicans on party affiliation is not only substantially greater than it was four years ago, but is the highest recorded during the past two decades.
The public continues to express more confidence in the Democratic Party than in the Republican Party as being able to bring about needed change, to govern in an honest and ethical way and to manage the federal government. The Democratic Party's advantages on these traits are much wider than during the last presidential campaign. Moreover, they remain about as large as they were just prior to the 2006 midterm election, in spite of rising public discontent with the Democrat-led Congress.
The voters' issues agenda also appears to benefit the Democrats. Along with Iraq, the economy, health care and education rate as the most important issues for voters. Compared with the 2004 campaign, fewer voters now place great importance on the issues that have animated Republican political unity in recent years – including gay marriage, abortion and terrorism.
Looking to the presidential election itself, the political climate appears to be affecting the morale of those in both parties. Democrats are more positive and more enthused than are Republicans. Since the beginning of the year, Democrats have closely followed campaign news at consistently higher rates than have Republicans, and somewhat greater proportions of Democrats say they have given a lot of thought to the presidential candidates.
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http://people-press.org/reports/display.php3?PageID=1202