Democrats will spend much of this midterm election year “finding their voice,” as liberal Democratic activists are fond of saying.

As I argued in this space last week, the possibility that some über-liberals might suddenly rise up and scare America with their rhetoric is a major reason that Democrats seem poised to snatch defeat from the jaws of victory. But it’s not the only reason.

Hillary Clinton’s imperious control over her party’s presidential politics will also become a stumbling block to the Democrats’ prospects for recapturing control of either house of Congress this November.

The 2008 contest for the Democratic nomination for president is already over, according to activists and even some news organizations. Just last week, ABC News released polling that implied Hillary’s the only one.

Ms. Clinton’s glittering favorable ratings among Democrats, even liberal ones, make it altogether a slam-dunk. Eighty percent of all Democrats and 86 percent of liberal Democrats hold favorable impressions of Her Highness.

Evidently no other Democratic candidates are even worthy of polling in light of numbers like those. Of course, ABC tested John McCain alone on the GOP side, but they know the GOP contest is just beginning. ABC just used McCain for balance to Hillary.

So what does Hillary’s 2008 hegemony have to do with 2006? In a nutshell, Hillary’s tight control of the primary field deprives this year’s Democratic congressional candidates of the energy and money that would result from a large field of prospective 2008 Democrats. And Ms. Clinton’s back-in-the-day persona leaves Democrats without a genuine newcomer or fresh face to meet the challenge evident from polls showing that large percentages of the electorate see the nation on the wrong track.

In times like these, voters want to turn to a new generation of leadership, something that voters won’t believe a Clinton or a Bush can provide. If this weren’t true, popular Florida Gov. Jeb Bush would be a GOP front-runner for 2008.

To understand the Democrats’ disadvantage, consider the GOP’s surplus of fresh, new faces. New-era candidates like Massachusetts Gov. Mitt Romney or Virginia Sen. George Allen will be embraced as a chance to turn the page and move to the next chapter in American politics.

Exciting new candidates like this can help congressional Republicans raise more money this year and energize grassroots activity in a way that candidates from the past cannot. Even John McCain has such an iconoclastic, maverick reputation that he’s still “fresh” in a world where Hillary is decidedly stale.

I’m convinced that the Republican field, already crowded, is not yet set. The electoral demand for “new” candidates is likely to drive some potential contestants from the “also-mentioned” category to full-fledged candidacies. The more, the merrier. Look for a few latecomers such as Govs. Bill Owens of Colorado or Mike Huckabee of Arkansas or Sens. Chuck Hagel of Nebraska or Lindsey Graham of South Carolina to fill out the field before fall.

Meanwhile, all this energy on the GOP side will be unmatched by Democrats. Frankly, it would be considered heresy to step forward and challenge Ms. Presumptive Nominee.

Of course, you say, a popular Hillary can help raise money and energize Democratic grassroots support. That’s true, but she’s only one person. And I’m not sure she’s the sort of hardworking, party-oriented candidate who’ll push through the fatigue to help one Democrat after another all year long, especially when she’s working for her own reelection. The new Republicans are younger and hungrier and have more to gain. They’ll outwork the New York senator and her complacent campaign every night of the year.

With Hillary sitting on her lead, Democrats will be stuck with even older, less inspiring generals leading the grass roots this fall. Whom do you want for your Jefferson Dinner: Kennedy, Gore or Byrd? Or the Democrats could risk pressing a 2012 or 2016 candidate like Illinois Sen. Barack Obama into the fray before his time, possibly compromising his future electoral prospects.

Hill is director of Hill Research Consultants, a Texas-based firm that has polled for GOP candidates and causes since 1988.