OLDERNESS, N.H. - The Democrats fueled the astonishing rise of Governor George W. Bush of Texas, who has a commanding lead in the New Hampshire primary. The Republicans weakened Vice President Al Gore, who is barely holding off his only challenger, Bill Bradley, the former New Jersey senator.
Politics, like Major League Baseball, is finally embracing interleague play.
For decades the two parties selected their candidates in long, ritual-rich nomination struggles that, for all their exposure to the outside world, might just as easily have been conducted in isolation chambers. Not in the 2000 campaign. For the first time in American politics, each party has conducted nomination battles whose contours, tone, rhythm and perhaps even outcome have been shaped by the other party.
The effect each party has had on the other has been imperceptible on a daily basis but undeniable over time. This unusual phenomenon has unfolded in four distinct waves.
First prominent figures, including the House minority leader, Richard A. Gephardt of Missouri, took themselves out of the Democratic presidential race. As they did, the assumption grew in the party that Gore was a safe bet to win the nomination.
Then Republicans, stung by their failure to remove President Clinton in the Senate impeachment trial and worried that the Democrats might extend their control of the White House by uniting behind a single candidate, flung their support behind a single candidate of their own, their enthusiasm for Bush growing with each poll that showed the Texas governor with a lead over the vice president.
Then the Democrats, troubled that their all-but-anointed standard bearer might be vulnerable in the November election, began to rethink their allegiance to Gore, providing an opening for Bradley.
Finally, Bradley exploited the uneasiness Democrats felt because of the strength shown by the Republican front-runner. Now a candidate who was an afterthought only a few months ago is only four percentage points behind Gore in the latest Boston Globe/WBZ poll.
Although the New Hampshire primary is still five months off, it is possible that Democratic support for Gore choked off a contest within the Republican Party that at one time showed every indication of being lively and provocative. But that process - the GOP rush to Bush - had the perverse effect of creating a contest in the Democratic Party where there hadn't been one.
The result is that each party, in an effort to respond to what is occurring in the other, has relinquished some of its control on its own nomination process. And the effect has been to crowd out political figures on the far fringes of each party, giving the Democratic left wing and the Republican right wing far less power over the nomination process than in earlier contests.
All these bounces - the rush to Gore, the flood to Bush, the retreat from Gore, the opening to Bradley - have served to reinforce the center of the political world. Bush's very appeal is his lack of strong ideological coloration. Gore and Bradley are both relative centrists, though Bradley has shown signs of a classic hard-court move: fake right, go left.
Overall, though, likely voters in both parties this election seem motivated less by ideology than by victory. They are far less interested in ''sending a message,'' as the late Governor George C. Wallace of Alabama used to say, than in sending a strong nominee to the general election. For the last third of a century, the nomination fights have been shaped in large measure by the extremes within the parties, with Republican primary voters more conservative and Democratic primary voters more liberal than the parties at large - and the electorate.
The course of the 2000 campaign so far reflects one of the lessons the two parties have stubbornly resisted until now: Elections are won in the middle.
The crossover effect of one party on the other, however, hasn't been confined merely to swings in candidate support. It shows up in the daily drizzle of news as well.
In the past, pressure on candidates muscling for advantage in nomination fights has come from within their own party. It was other Democrats, for example, who set in motion the plagiarism furor that forced Senator Joseph R. Biden Jr. of Delaware from the 1988 race. But this time some of the fervor fueling the contretemps over whether Bush ever used cocaine came from the Democrats. One such demand for a straight answer from Bush came from the Senate minority leader Thomas A. Daschle, a South Dakota Democrat.
This year political power has been like a pinball, bouncing from one end of the political playing surface to the other - the ricochets powerful, the impact inescapable, the implications unpredictable. That's why the fifth bounce - the trajectory begun, the ultimate course unknown - is worth tracking.
