WHAT do Pat Buchanan and George Bush have in common, besides belonging to the same political party and coveting the same high office? Both understand a key factor in winning elections for Republicans: evangelical Protestants.Mr. Buchanan, a Roman Catholic and leading conservative spokesman, regularly appears in evangelical forums and cultivates Christian activists along with other elements of the New Right. President Bush, an Episcopalian and hardly a favorite of the New Right, nevertheless pays attention to evangelicals and delivers on litmus test issues such as abortion and family leave. A third competitor, David Duke, also understands the importance of evangelicals, having carried the white evangelical vote in the Louisiana gubernatorial runoff. But his racist appeals will limit his ability to mobilize evangelicals nationally. But haven't evangelicals faded out of politics with the dissolution of the Moral Majority and Pat Robertson's poor showing in 1988? No. While some of the initial actors have declined in prominence, they have been replaced by more sophisticated organizations and leaders, and many evangelical activists have moved into regular party politics. OF far more significance, however, a fundamental change is occurring in the connection between the Bible and the ballot box in the United States. Political alignments are shifting away from traditional ethnic conflict between religious traditions - Protestant, Catholic, and Jewish - and toward ideological conflict between believers and less religious people. This pattern is strongly reminiscent of European politics, where religious advocates of "order and tradition" and secularist forces of "rights and r eform" have routinely struggled over cultural questions, such as the place of religion in public life, control of schools and curricula, sexual morality, and the role of women. Central to these changes are dramatic shifts in US religion itself. Foremost is the new prominence of secular people, many of whom are hostile to traditional social arrangements. Closely related to this is the declining membership of mainline Protestant denominations. Bitter internal disputes in these churches have muddled voices and clouded vision in politics. Similar internal divisions have appeared among Catholics as well. At the same time, conservative Protestants are increasing in number and their w ealth serves the cause of "traditional values." These religious shifts have direct political implications. Secularists tend to favor the social-issue liberalism of the Democratic Party, as does the "peace and justice" wing of mainline Protestantism. These groups have joined liberal Catholics, Jews, and African-Americans in the Democratic coalition. On the other hand, traditionalist Catholics and evangelical Protestants, as well as the traditionalist wing of mainline Protestantism, find the social and foreign-policy conservatism of the GOP attractive. If this process continued to its logical conclusion, the GOP could increasingly become an alliance of religious traditionalists and orthodox believers from many backgrounds, while the Democrats could become a coalition of secularists and the religiously heterodox. But many factors could inhibit the full development of such alignments: historic animosities between churches, a sour economy, a prolonged foreign policy crisis. But the raw materials for such coalitions are increasingly available and the incentives for exploiting them are becoming stronger. Already, presidential candidates in both parties take pains to be on the appropriate side of the cultural foment that besets America. For Republicans this means cultivating the "religious vote," and for Democrats it means catering to groups often hostile to "the old-time religion." This new connection between the Bible and the ballot box will surely influence next year's Republican primaries, play an important role in the 1992 general election, and loom larger in 1996.