Sen. Lamar Alexander (Tenn.) will not make the mistake of prematurely assuming he has wrapped up victory in Thursday’s leadership race against Sen. Richard Burr (N.C.) to become the Senate Republicans’ conference chairman.

A year ago he was brutally disappointed when supporters who had already committed to him defected to Sen. Trent Lott (R-Miss.) and thus denied the Tennessean the post of minority whip. A similar reversal now is not out of the question, so neither candidate is likely to count his chickens until they hatch at about 9:30 Thursday morning.

Still, Alexander, at least preliminarily, appears to have secured the support of 25 of his 49 Senate colleagues, enough to put him over the top and place him third in the GOP leadership behind Minority Leader Mitch McConnell (Ky.) and incoming Minority Whip Jon Kyl (Ariz.).

The race is highly significant because it will usher in a new generation of Republican leaders. Lott’s victory a year ago re-established the supremacy of the incumbent generation. Now his decision to retire allows the party to present a new face to the country.

For sure, Alexander is well-known, having already served as education secretary in the administration of President George H.W. Bush, and having also run twice for the GOP’s presidential nomination.

Nevertheless, he, almost as much as Burr, would give the top of the party in the Senate a new look, shimmering with fresh ideas and energy. Few people on Capitol Hill or elsewhere doubt that the party needs these, among other things, to recapture voters’ allegiance. Certainly the GOP faces an excruciatingly tough battle defending 23 Senate seats next November, and is expected to suffer a net loss of seats.

As conference chairman, Alexander or Burr will have primary responsibility for shaping the Senate Republicans’ message, and they could not take on that task at a more difficult and demanding hour.

Neither is likely to be punished if, as expected, Republicans go down to defeat 11 months from now; although they must do their best to limit immediate setbacks, it is probably recognized (if not acknowledged) that the primary task of the new leadership slate will be to build the party back to fighting fitness for the elections of 2010 and 2012.

Thus, Alexander or Burr will likely be one of the faces of the party for years to come. The former is considered more of a centrist, so their colleagues are today choosing which facet of Republicanism should be presented to voters for the next half-decade or so.

It’s a weighty choice.