Richard Diebenkorn, Ocean Park No. 38, oil on canvas, 1971

The market for museum giving is in a doldrums: Recent changes to the tax code have encouraged collectors to horde their treasures instead of donating them. That’s bad timing for the Phillips Collection, which has been attempting to stake out a new course. “Degas to Diebenkorn: The Phillips Collects” hopes to rebrand the museum’s image from sturdy-but-static to evolving-and-dynamic, and it is as much a pitch to potential donors as it is a proclamation of the successes of a “vigorous acquisitions program,” as the show’s press release puts it. Among other things, the exhibit proves just how hard that course will be. With financial incentives for giving reduced, the museum has to fight harder for gifts, and the proof is on the gallery walls: Works once under the more secure category of “fractional gift” have been replaced with the fuzzier “promised gift” throughout the show.

But despite the transformed giving environment, “Degas to Diebenkorn” is of a piece with other changes over the last few years at the red brick house on 21st Street. The Phillips celebrated a dramatic expansion in 2005 that added five stories and 3,000 square feet of gallery and studio space. That same year, it closed a $29 million capital ­campaign—its first ever—some $2 million over its target and two years ahead of schedule. Last year’s announcement that director Jay Gates would retire sees the Phillips Collection’s fifth director departing as a hero. His replacement, Dallas Museum of Art senior curator Dorothy Kosinski, has said she wants to work with living artists—something the museum hasn’t traditionally done. Among the 100 or so new additions in “The Phillips Collects” are works by 28 artists new to the museum, several of them living or only recently dead.

That pace may start slowing: A tweak to the tax code in 2006 means that donated artworks are now assessed at their lowest value over time, providing collectors with less of an incentive to give. (Collectors also now take a bigger hit on estate taxes.) But it’s not just the tax code that makes Kosinski’s stated goal tricky to fulfill. The collection is built on founder Duncan Phillips’ singular modernist vision. The Phillips can boast some serious works brought together by that idea, from Pierre-Auguste Renoir’s Luncheon of the Boating Party (1880-81) to Pierre Bonnard’s extravagant The Open Window (La Fenêtre Ouverte) (1921) to three gorgeous interiors by Edouard Vuillard. But the museum’s mission statement is its own obstacle: Modernism is dead, so how do you expand a modernist collection?

One answer during Gates’ tenure was to acquire more of the same. The promised gift of Vuillard’s Interior With a Red Bed (1893) is an elegant solution in this regard: As a cousin to the Vuillards that the museum already owns, the painting enriches one of the best caches of a single artist’s work at any museum in the region. Edgar Degas’ La Répétition au Foyer de la Danse (1870-72) is another important acquisition, and the Phillips has bolstered its holdings of works by Paul Klee (from 13 to 15), Arthur Dove (48 to 54), and Pierre Bonnard (30 to 32, plus one promised gift).

The museum made some surprising choices within that strategy. One high-­ceilinged, sunlit gallery pairs large paintings by Richard Diebenkorn and Elizabeth Murray. Diebenkorn already is familiar to the collection, but Ocean Park No. 38 (1971) is a stunning addition, a key work from his most important series of abstractions. (In fact, with this painting and a newly acquired 1960 Bay Area figurative movement piece by Elmer Bischoff, Figures: Back and Profile, the Phillips is poised to be the local source for the best modernist art from the West Coast.) Opposite the roughly 8-by-7 foot Diebenkorn monster is a provocative choice: Jazz, a 2001 painting by the late Elizabeth Murray. It’s not her strongest work: The canvas is typically funky and fragmented, but her signature cartoonish biomorphs are muted and less electric than earlier works, whose imagery was more sexual, frank, and visceral. The two paintings together make for an odd but interesting couple: Diebenkorn, studied and introverted; Murray, distracted and flirty. More than monumental size recommends that the two paintings be kept near each other; it’s an awkward date, but one that promises intriguing conversation.

Another gallery, showcasing recent photography additions, is a startling, beautiful room featuring surprise signature accomplishments like Henri Cartier-Bresson’s powerful 1947 portrait of William Faulkner and Paul Strand’s iconic 1915 photo of Wall Street. The three newly acquired photographs by Aaron Siskind, dating from 1954 to 1982, bridge modern photographic techniques and a sense of composition that recalls abstract expressionist Robert Motherwell.

If the Phillips eventually can’t find more of the same, another approach is to find more like the same. But finding relevant contemporary art with a modern sensibility that can stand among giants is problematic. An example is Three Masks, a 2006 painting by Susan Rothenberg. On its face, it’s a sensible inclusion: It’s an abstract-leaning piece emphasizing color field and composition, and masks are art-historically inclined subjects. However, abstraction of this sort isn’t pushing the envelope in the 21st century, and Rothenberg can’t hang with the other three paintings anchoring the gallery by Bischoff, Milton Avery, and Helen Frankenthaler, artists who guided and developed the parameters of expression through abstract painting. These artists are in conversation with one another; Rothenberg is just repeating what they’re saying.

With Rothenberg and other living artists whom the museum courts, the Phillips Collection runs into a wall. Those like Sean Scully who advance the notion of a continued modernist dialogue absent postmodernism (and the work that follows today) are scarce. (Five new Scullys grace the exhibit, though.) New developments in art and theory will continue to winnow the field. This isn’t a problem facing the Phillips alone: The venerable Museum of Modern Art looks increasingly stretched by its mission to proffer a comprehensive narrative including both the modernist and the contemporary. But with the field for giving shrinking as collectors tighten their purses, the Phillips has its work cut out for it. That the museum seeks to promote the continuing relevance of Duncan Phillips’ vision is a given. That it hopes to continue that vision is a bold and welcome gesture. Doing so in a coherent fashion—and without bending artworks to an inappropriate purpose—may prove a harder obstacle than anything that the tax code can throw at them.