Let $\lambda$ be the space of symmetric functions, and let $V\ sb
k$ be the subspace spanned by the modified Schur functions $\{S\sb
\lambda[X/(1-t)]\}\sb {\lambda\sb 1\leq k}$. We introduce a new family
of symmetric polynomials, $\{A\sp {(k)}\sb \lambda[X;t]\}\sp
{\lambda\sb 1\leq k}$, constructed from sums of tableaux using the
charge statistic. We conjecture that the polynomials $A\sp {(k)}\sb
\lambda[X;t]$ form a basis for $V\sb k$ and that the Macdonald
polynomials indexed by partitions whose first part is not larger than
$k$ expand positively in terms of our polynomials. A proof of this
conjecture would not only imply the Macdonald positivity conjecture,
but also substantially refine it. Our construction of the $A\sp
{(k)}\sb \lambda[X;t]$ relies on the use of tableau combinatorics and
yields various properties and conjectures on the nature of these
polynomials. Another important development following from our
investigation is that the $A\sp {(k)}\sb \lambda[X;t]$ seem to play
the same role for $V\sb k$ as the Schur functions do for $\lambda$. In
particular, this has led us to the discovery of many generalizations
of properties held by the Schur functions, such as Pieri-type and
Littlewood-Richardson-type coefficients.