For both parties, it was like a premature preview of 2008 presidential politics.

No sooner had Senate Majority Leader Bill Frist reversed himself to back expanded federal support of embryonic stem cell research than Republican conservatives went on the attack.

“Senator Frist’s public backing of this horrific science is being deeply felt across middle America and, most importantly, at the grass-roots,” said Tamara Scott, Iowa state director of Concerned Women for America.

Her criticism was similar to that Sen. John McCain attracted two months ago when he brokered an agreement to head off a Senate showdown on curbing Democratic efforts to filibuster conservative judicial nominees.

But Republicans are not the only ones with internal divisions. A similar spat erupted just days earlier on the Democratic side.

Liberal groups chided Sen. Hillary Rodham Clinton and other prospective 2008 Democratic candidates for wooing the Democratic Leadership Council, the centrist group that spawned Bill Clinton’s 1992 candidacy.

“The Democratic Party is in the throes of a battle for its soul,” said liberal activist David Sirota, who heads the new Progressive Legislative Action Network. The choice, he said, is between the DLC, whom he labeled “a tiny cadre of conservative-leaning elitists” and progressives willing “to stand up for America’s middle class.”

He also hit 15 House Democrats for backing the Central American Free Trade Agreement.

These divisions are likely to play out over the next three years as the parties seek to define themselves for the most wide-open presidential election in decades.

With President Bush retiring and Vice President Dick Cheney not running to succeed him, 2008 will be the first race since 1952 with neither the incumbent president nor vice president.

More than a dozen hopefuls have taken steps toward presidential bids by forming political committees, adding staff and visiting states like Iowa, New Hampshire, Michigan and South Carolina that hold early caucuses and primaries.

Republicans seem certain to face a choice between candidates who stress the social agenda of religious conservatives who have controlled the party for 25 years and one or more centrist rivals.

The first group includes Sens. George Allen of Virginia and Sam Brownback of Kansas. The second group might include Gov. George Pataki and former Mayor Rudy Giuliani of New York.

Some potential candidates may not fit conveniently into one camp. Until he angered social conservatives by revising his stem cell position, Frist seemed to appeal more to that wing of the party.

Gov. Mitt Romney of Massachusetts has been hardening his stance against abortion rights. And though McCain correctly calls himself a “pro-life, pro-defense conservative,” his efforts to work with Democrats and to oppose some Bush tax cuts have cast him in the centrist faction.

It’s hardly coincidental that conservatives cited a statement from their allies in Iowa, which has held the first caucuses since the 1970s and where religious conservatives control the GOP.

Until 2000, however, centrist Republicans won every Iowa caucus contest. But McCain bypassed the 2000 caucuses out of concern about religious conservatives. Many believe he’d adopt a similar strategy in 2008.

Meanwhile, Democrats will be deciding between candidates in the mold of the party’s last two presidents, Jimmy Carter and Bill Clinton, and more traditional liberals, many of whom favor a more confrontational stance on issues like the Iraq war and foreign trade.

Even more than Mr. McCain’s efforts to span the GOP spectrum, Mrs. Clinton’s attempt to adopt a more centrist stance has complicated the prospect of a center-left clash within the Democratic Party.

Though the former first lady would have substantial liberal backing if she runs, her support of the Iraq war may attract an opponent on her left, possibly Sen. Russ Feingold of Wisconsin or unsuccessful 2004 nominee John Kerry.

But just as Iowa Republicans have rarely favored the more conservative candidate, so have its Democrats seldom picked the most liberal hopeful. In 2004, despite substantial anti-war sentiment, the top anti-war candidate, former Gov. Howard Dean of Vermont, lost to Kerry.

How these races play out may depend ultimately on who runs and on conditions at the time of the 2008 primaries and caucuses.

But the pattern of last week’s clashes is likely to seem quite familiar by the time the 2008 nominees are chosen.