Matt Grossmann is a political scientist at Michigan State University and author of “Artists of the Possible: Governing Networks and American Policy Change Since 1945.”

Conservatives in Congress are the prime suspects in Washington’s dysfunction. Veteran congressional watchdogs Norman Ornstein and Thomas Mann called the previous session the “worst Congress ever,” and they did not hold back in assigning blame: “The Republicans are the problem,” they said. After a fruitless government shutdown last fall, even House Speaker John Boehner lashed out at conservative groups and passed bills over the opposition of his caucus.

In response, conservatives make two simple claims: Most policies under debate are liberal, and Republican leaders sacrifice conservative principles when they compromise. History shows they are right on both counts.

I combed through hundreds of history books covering American public policy since 1945, tracking the most significant domestic policy changes that made it into law and the actors that historians credit for those changes. Of the 509 most significant domestic policies passed by Congress, only one in five were conservative, in that they contracted the scope of government funding, regulation or responsibility. More than 60 percent were liberal: They clearly expanded government. The others offered a mix of liberal or conservative components or took no clear ideological direction. When significant policy change occurs in the executive branch, it is even less likely to be conservative; only 10 percent of the executive orders and agency rules that policy historians cited were conservative.

Even labeling as conservative policy government expansions in pursuit of conservative goals, such as traditional values or tougher sentencing, makes little difference in this conclusion; few significant policy changes fall into this category, though we hear about them often in campaigns.

Not surprisingly, liberals play a greater role in bringing about new policy. Historians credited twice as many liberal interest groups as conservative groups with policy change. Democratic politicians also played more frequent roles. The most active Republicans, such as President Richard Nixon and Sens. Jacob K. Javits and Bob Dole, supported mostly liberal and mixed policy changes, rather than conservative changes. Republican involvement in policy has meant compromising with liberals to expand government or trading contraction in one place for expansion in another.

There is a good reason why conservatives are often charged with obstruction. When government is more active, it is usually moving policy to the left. When Congress has doubled its normal productivity, many more liberal laws pass but not necessarily more conservative laws. There was only one session of Congress, the two years after the Republican takeover in 1994, that was both active and conservative, but it did not last. Under President Ronald Reagan, the executive branch made more conservative policy changes only during the first two years of his presidency. Productive policymaking means more domestic spending, more business regulation and wider government responsibility.

These trends are not a product of the unique perspective of policy historians. Other scholars’ analyses of major laws identified by journalists find similar results but even less conservative lawmaking. Liberal policies are self-reinforcing because they create beneficiaries who act as constituencies for their continuation and expansion. Policy debates center on what additional actions government should take, not whether to discontinue existing roles.

Tea party godfather Jim DeMint left a prime policymaking role in the Senate last year to run the Heritage Foundation, calling it a “big promotion.” Indeed, DeMint gains little from cutting deals in Congress. The view that normal legislating and bipartisan compromises lead to expanded government is no tea party illusion; it is an accurate reading of the past 70 years.

To be sure, the conservative failure to shrink government does not imply constant liberal victory. No change is still the most likely outcome in any legislative battle. Obstruction is strategic: shared powers, checks and balances and multiple veto points make policy change difficult to achieve. Some liberal policies, such as the minimum wage, lose effectiveness if they are not regularly updated. A few conservative successes, particularly in tax policy, can overwhelm more numerous liberal laws.

But the federal government has continually expanded its role in education, civil rights, the environment and health care — and Republican presidents have played large roles in this. Nixon entrenched the Great Society and oversaw the environmental revolution. Reagan was less active domestically but signed more government expansions than contractions. President George H.W. Bush brought us landmarks such as the Americans with Disabilities Act, and his son brought us No Child Left Behind and a new Medicare entitlement.

This history does not bother some Republicans, who see opportunities to fashion new ideas and bargain in pursuit of conservative objectives. But even past policymaking designed to promote markets, safeguard morality and protect the homeland usually expanded government. If contraction is the goal, a positive policy agenda is unlikely to succeed. Conservatives like DeMint mean it: Their priority is a smaller government that does less — and it may not be achievable.

The arc of the policy universe is long, but it bends toward liberalism. Conservatives can slow the growth of government but an enduring shift in policy direction would be unprecedented. History shows that a do-nothing Congress is a conservative’s best-case scenario.