The Washington Pension Union (WPU) fought to give the state of Washington one of the most generous welfare systems in the nation. Leaders of the Washington Commonwealth Federation (WCF), a group that worked within the Democratic Party to elect left-liberals to state and local offices, created the Washington Old Age Pension Union in 1937 in an effort to recruit new members. The new group (which changed its name to the Washington Pension Union in 1944) soon grew much more powerful than the WCF, which dissolved in 1945. By 1940, the WPU, with five of its leaders in the state legislature, was clearly the largest left-wing organization in the state.

The WPU had two purposes. It was a "union" for people on public assistance that informed them of their rights, helped them fill out application forms, and represented them in appeals cases if they were denied aid. The WPU also lobbied for more liberal eligibility criteria and benefit levels for public assistance. Local chapters concentrated on the first function, while the state leadership focused on the second.

The WPU scored its first major victory in 1940 with the passage of Initiative 141, which expanded social security to ensure that state residents over 65 had an income of at least $40 per month. Divisions between the WPU and proponents of the Townsend pension plan doomed the next two WPU measures (Initiative 151 in 1942 and Initiative 154 in 1944). The WPU board of trustees fired President N. P. Atkinson in 1942 for losing the Initiative 151 campaign and for failing to adhere to the Communist Party line on foreign policy issues. The new president, William Pennock, re-energized the WPU. He formed a sister agency, the Washington Aid to Dependent Children Union, to organize mothers on public assistance. Pennock also mended fences with the Townsendites and put together an impressive coalition in favor of Initiative 172, which the voters approved in 1948. This measure raised the minimum income level for senior citizens to $60 per month and guaranteed free health care to everyone receiving any form of public assistance.

The WPU's political fortunes declined after 1948. Implementing Initiative 172 consumed fully half of the state's budget in 1949 and rapidly turned Washington's budget surplus into a substantial deficit. When Governor Langlie cut welfare spending in 1950, the WPU filed Initiative 176 to raise seniors' minimum income to $65 per month and to provide free telephone and laundry service to all those on public assistance. Langlie retaliated by drafting Initiative 178 to abolish the minimum income guarantee and to subject medical benefits to the approval of a panel appointed by the governor. Langlie accused the WPU of being a "communist front" trying to bankrupt the state. The WPU's vigorous denunciation of the Korean War and its calls for the United States to make peace with the Soviets seemed to give credence to Langlie's accusations. Voters approved Langlie's measure, while solidly rejecting the WPU's. In 1952, the WPU's attempt to revive a minimum income level for seniors, Initiative 184, failed. Nonetheless, even after cuts in welfare spending in the 1950s, Washington's welfare system was still quite liberal relative to other states'.

In 1953, U.S. Attorney General Harry Cain declared that the WPU was a subversive organization and ordered Pennock to testify before the U.S. Senate Subversive Activities Control Board. Pennock admitted he was a member of the Communist Party in his testimony. On August 2, 1953, just days before he was scheduled to testify again, Pennock committed suicide by taking too many barbiturates. The WPU spent the remainder of its existence fighting government lawsuits aimed at dissolving the group. Membership plummeted. Although the WPU helped the CIO promote the unsuccessful Initiative 200 to increase unemployment payments in 1954, the group could no longer collect enough signatures to get its own proposals on the ballot. In 1961 the increasingly ineffectual WPU voluntarily dissolved.

Even after its dissolution, the WPU was still involved in legal cases against the Subversive Activities Control Board. The Control Board ruled that, even though the WPU no longer existed or conducted business, many of its former members still saw each other socially in a manner that would make it possible to revive the WPU. As such, the Control Board required the WPU to continue to register as a Communist front organization as it had been required to do beginning on April 14, 1959 under the Subversive Activities Control Act of 1950, even though the Union no longer existed. The former officers of the WPU fought this decision in court in early 1962, where the Board determined that the WPU was required to continue to register as a Communist front organization.

The main officers of the WPU were: N. P. Atkinson, President, 1937-1942; William Pennock, Executive Secretary, 1937-1943 and President, 1943-1953; Charles H. Fisher, Vice-President, circa 1944-1953 and President, 1953-1961, and Educational Director; Mable Conrad, Executive Secretary, 1943-1961; Thomas Rabbitt, Vice-President, 1938-1961; Lenus Westman, Vice-President, 1937-?; E. L. Pettus, Vice-President, 1937-circa 1944; and Louise Pennock, Vice-President, 1953-1961.